I am getting this error for below StandardSetController code

14:28:20.105 (105857000)|EXCEPTION_THROWN|[13]|System.QueryException: Non-selective query against large object type (more than 100000 rows). Consider an indexed filter or contact salesforce.com about custom indexing.

Even if a field is indexed a filter might still not be selective when:

1. The filter value includes null (for instance binding with a list that contains null)

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public class duplicateSMSFinder{

 public static string processNo;

 public ApexPages.StandardSetController setCon {
 
        get {
                  
              if(setCon == null && processNo !=null)
                {
                try{
                      system.debug(processNo+'ddddddddd');
                      setCon = new ApexPages.StandardSetController(Database.getQueryLocator([SELECT id,
                                                    Email,
                                                    Customer_Mobile__c,
                                                    Mobile_Number__c from
                                                    lead where
                                                       Customer_Mobile__c=:processNo limit 1]));
                      
                   }catch(exception e){}  
                      
                }//if
               
            return setCon;
        }     
        set;
    }//construcor

    // Initialize setCon and return a list of records
    
    public List<lead> getleadLists(string pSMSNO) {
    processNo=pSMSNO;
     if(setCon != null){
     return (List<lead>)setCon.getRecords();    
     }
     else{return null;}
    
    }
                
} //end of class

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