Hi,
I have added try catch in your code please make changes accordingly but i thing there is problem in your query that's why this exception occur.
By Try catch you can only avoid exception in your Vf page.

public with sharing class ObjectQueryController6 {
    public List<Floor_Plan_Mapping__c> conts{get;set;}
    public Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
    private Map<String, String> fldAPINameMap = new Map<String,String>();
    
    
    //Private set on SelectOption is so that values can be set from the controller.  
    public List<SelectOption> objectNames{public get; private set;}
    public String selectedObject {get; set;}
    public String selectedField {get;set;}
    public String selectedField1 {get;set;}
    public String selectedField2 {get;set;}
    public String selectedField3 {get;set;}
    public String selectedField4 {get;set;}
    public String selectedField5 {get;set;}
    public String Additionalfield1 {get;set;}
    public String Additionalfield2 {get;set;}
    public String Additionalfield3 {get;set;}
    public String Additionalfield4 {get;set;}
    public String Additionalfield5 {get;set;}
    public String Additionalfield6 {get;set;}
    
    public String selectedFieldAPI {get;set;}
    public String selectedFieldAPI1 {get;set;}
    public String selectedFieldAPI2 {get;set;}
    public String selectedFieldAPI3 {get;set;}
    public String selectedFieldAPI4 {get;set;}
    public String selectedFieldAPI5 {get;set;}
    public String AdditionalfieldAPI1 {get;set;}
    public String AdditionalfieldAPI2 {get;set;}
    public String AdditionalfieldAPI3 {get;set;}
    public String AdditionalfieldAPI4 {get;set;}
    public String AdditionalfieldAPI5 {get;set;}
    public String AdditionalfieldAPI6 {get;set;}
    

    
    private List<SelectOption> fields;
    Public string criteria{get;set;}
    transient public List<sObject> results{get;set;}
    public List<sObject> result1s{get;set;}
    public boolean queried {get;set;}
    
    // TO DO:
    // - Error handling when user selects ID or Name fields to query (Any Duplicate Fields)
    // - Handle queries of fields that are different than text
    // - Allow the selection of multiple fields
    // - Allow Mass Update of fields on queried records
    // - Insert new records
    
       public List<SelectOption> getObjFields() {     
       try{
        fields.clear();
        if(queried == null){queried = false;}
        system.debug('Page loading, Queried is: '+queried);
        
        // If an object was not selected yet, just populate the text 'Select object first'
        if(selectedObject == NULL){
            fields.add(new selectOption('Select Object First', 'Select Object First'));
        } 
        
        // if object was selected, create a list of the object's fields
        else { 
              fields.add(new selectOption('', '- None -'));           
            system.debug('$$$$$' + selectedObject);
            
            //We get a string of the sOjbects using the schemaMap.
            Map <String, Schema.SObjectField> fieldMap = schemaMap.get(selectedObject).getDescribe().fields.getMap();
            
            for(Schema.SObjectField sfield : fieldMap.Values())
            {
                schema.describefieldresult dfield = sfield.getDescribe();
                String fieldName = dfield.getName();      
                fields.add(new SelectOption(fieldName, dfield.getLabel()));
                fldAPINameMap.put(fieldName, dfield.getname());
            }
        }
        return fields;
      }
      catch(Exception e){
        system.debug('Exception at Line Number'+e.getLineNumber+'because of '+e.getMessage());
      }
    }
    
    // Constructor - this method will run when the controller is instantiated
    public ObjectQueryController6(){
        objectNames = initObjNames();
        fields = new List<SelectOption>();
        // List<sObject> results = new List<sObject>(); 
        String Criteria;
    }
    
    // Populate SelectOption list -
    
    // find all sObjects available in the organization
    
    private List<SelectOption> initObjNames() {
    try{
        List<SelectOption> objNames = new List<SelectOption>();
 
        List<String> entities = new List<String>(schemaMap.keySet());
        entities.sort();
        for(String name : entities)
            objNames.add(new SelectOption(name,name));
           
        return objNames;
    }
    catch(Exception e){
      system.debug('Exception at Line Number'+e.getLineNumber()+'because of '+e.getMessage());
      return null;
    }
    
    }
    
    //Instantiate the list cast records.  We pull id, Name and other needed fields for our pageBlockTable in our VF page
    
    public void btnPerformQuery(){
    try{
        results=new List<sObject>();
        String Additionalfields = '';
        if(Additionalfield1 != null) {
        Additionalfields += fldAPINameMap.get(Additionalfield1)+',';
        }
        if(Additionalfield2 != null) {
        Additionalfields += fldAPINameMap.get(Additionalfield2)+',';
        }
       if(Additionalfield3 != null) {
       Additionalfields += fldAPINameMap.get(Additionalfield3)+',';
       }
       if(Additionalfield4 != null) {
       Additionalfields += fldAPINameMap.get(Additionalfield4)+',';
       }
       if(Additionalfield5 != null) {
       Additionalfields += fldAPINameMap.get(Additionalfield5)+',';
       }
       if(Additionalfield6 != null) {
       Additionalfields += fldAPINameMap.get(Additionalfield6)+',';
       }
        string queryFields = Additionalfields +''+fldAPINameMap.get(selectedField)+','+fldAPINameMap.get(selectedField1)+','+fldAPINameMap.get(selectedField2)+','+fldAPINameMap.get(selectedField3)+','+fldAPINameMap.get(selectedField4)+','+fldAPINameMap.get(selectedField5)+'';      
        string queryStr1 = 'SELECT createdDate,lastModifiedDate,'+queryFields+'  FROM '+selectedObject+' WHERE '+fldAPINameMap.get(selectedField)+' IN (\'' + String.join( criteria.split( ',' ), '\',\'' ) +'\') limit 1 ';
        system.debug('queryStr1-='+queryStr1);
        selectedFieldAPI = fldAPINameMap.get(selectedField);
        selectedFieldAPI1 = fldAPINameMap.get(selectedField1);
        selectedFieldAPI2 = fldAPINameMap.get(selectedField2);
        selectedFieldAPI3 = fldAPINameMap.get(selectedField3);
        selectedFieldAPI4 = fldAPINameMap.get(selectedField4);
        selectedFieldAPI5 = fldAPINameMap.get(selectedField5);
        AdditionalfieldAPI1=fldAPINameMap.get(Additionalfield1);
        AdditionalfieldAPI2=fldAPINameMap.get(Additionalfield2);
        AdditionalfieldAPI3=fldAPINameMap.get(Additionalfield3);
        AdditionalfieldAPI4=fldAPINameMap.get(Additionalfield4);
        AdditionalfieldAPI5=fldAPINameMap.get(Additionalfield5);
        AdditionalfieldAPI6=fldAPINameMap.get(Additionalfield6);
        results = database.query(queryStr1);
        result1s = results;
        System.Debug('results-='+results);
        queried = true;
        system.debug('Query performed, Queried is: '+queried);
    }
catch(Exception e){
   system.debug('Exception at Line Number'+e.getLineNumber()+'because of '+e.getMessage());
}    
        
    }
    
   
    List<Floor_Plan_Mapping__c > accountList = new List<Floor_Plan_Mapping__c>();

    public void Save()
    {  
        try{
     accountList.add(new Floor_Plan_Mapping__c(Object__c=selectedObject,Edition__c =selectedField,Opportunity_Id__c =selectedField1,Opportunity__c=selectedField2,Stage__c =selectedField3,Banner_Name__c=selectedField4, Stand_no__c=selectedField5,
                                                  Additional_field_1__c=Additionalfield1,Additional_field_2__c=Additionalfield2,Additional_field_3__c=Additionalfield3,Additional_field_4__c=Additionalfield4,
                                                  Additional_field_5__c=Additionalfield5,Additional_field_6__c=Additionalfield6));

       
       List<sObject> objects = new List<sObject>();
       objects.addAll((List<sObject>)(accountList)); 
            if (objects == null  && objects.isEmpty())
            {
             ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Please enter Account Name'));
            }
            if (objects != null  && !objects.isEmpty())
           {
           ApexPages.addmessage(new ApexPages.message(ApexPages.severity.CONFIRM,'Mapping Saved Sucessfully...'));
           }
       insert objects;
        system.debug('results-='+result1s);  
        }
        catch(Exception e){
            system.debug('Exception at Line Number'+e.getLineNumber()+'because of '+e.getMessage());
        }
    }
}



I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks and Regards,
Deepali Kulshrestha