Hey  Guys, 

I have created a visualforce page which displays the list of records which meet the critiriea on the page. Along with this , there are some fields  which are performaing some calculations. I am trying to display error message when it doesn't mee the critirea. It is working fine when there is no calculation field but when I include the calculation field , it gives me this error. In short, If I remove the Highlighted line in apex code it works fine but I do want to include that line also. Please help.
Attempt to de-reference a null object
Error is in expression '{!FetchSPRec}' in component <apex:commandButton> in page trial_rfp_page

Calculation field :- MaxImpression

Apex code:

public class Fetchsiteplacement12{
public Site_Placements__c  sp{get;set;}
    public List<Site_Placements__c > spRec{get;set;}
    String matchString;
    String matchString1;
    public integer NumberOfSitePlacements {get;set;}
    public decimal SumOfImpressions{get;set;}
    public decimal MaxImpressions{get;set;}
   
    public Fetchsiteplacement12(){
        sp=new Site_Placements__c ();
        spRec = new List<Site_Placements__c>();
        matchString = '';
        matchString1 ='';
        }
       
            public void FillAggregates()
                 {
                       NumberOfSitePlacements = [ Select count()
                       FROM Site_Placements__c where ((Device_Type__c like :matchString) AND (Auto_Play__c like :matchString1))];
          
                       SumOfImpressions = (decimal)([ SELECT sum(Total_Impresions__c)
                       FROM Site_Placements__c Where ((Device_Type__c like :matchString) AND (Auto_Play__c like :matchString1))][0].get('expr0'));
                      
                       MaxImpressions = SumOfImpressions/NumberOfSitePlacements ;
                 }
  
 
        public void FetchSPRec(){
        matchString = '%'+sp.Device_Type__c+'%';
        matchString1 = '%'+sp.Auto_Play__c+ '%';
      
      
      
        spRec=[select Name, Site_Placement_ID__c, Site_Level_Max_Impressions__c from Site_Placements__c where
        ((Device_Type__c like :matchString) AND (Auto_Play__c like :matchString1) ) ORDER BY Site_Level_Max_Impressions__c DESC limit 10];
      
       if(spRec.size() == 0)
                  {
                   Apexpages.addMessage(new ApexPages.Message(ApexPages.Severity.INFO,''+'No records to Display"'));
                  }
                  FillAggregates();
                  }
      
       public pagereference CancelSPRec(){
    
    PageReference page = new PageReference('https://cs1.salesforce.com/a36/o');
    page.SetRedirect(true);
    return page;
    }}

VisualForce page:-

<apex:page controller="Fetchsiteplacement12" tabStyle="Site_Placements__c" sidebar="True">
    <apex:form >
        <apex:pageBlock >
        <apex:messages />
            <apex:pageBlockButtons location="Both">
         
            <apex:commandButton value="Fetch" action="{!FetchSPRec}"/>
            <apex:commandButton value="Cancel" action="{!CancelSPRec}"/>
            </apex:pageBlockButtons>
           
            <apex:pageBlockSection title="Please select the Critirea">
            
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value="Auto Play" for="autoplay"/>
                    <apex:inputField value="{!sp.Auto_Play__c}" id="autoplay"/>
                </apex:pageBlockSectionItem>
               
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value="Device Type" for="devicetype"/>
                    <apex:inputField value="{!sp.Device_Type__c}" id="devicetype"/>
                </apex:pageBlockSectionItem>
               
              
              <apex:pageBlockSection Title="Calculations">
          
                 <apex:pageBlockSectionItem > Number of site placements : {!NumberOfSitePlacements}<br/></apex:pageBlockSectionItem>
                 <apex:pageBlockSectionItem > Max Impressions : {!Maximpressions}<br/></apex:pageBlockSectionItem>
                 </apex:pageBlockSection>
            
              <apex:pageBlockSection title="Results">
            <apex:pageBlockSectionItem >
                <apex:pageBlockTable value="{!spRec}" var="site" >
                    <apex:column value="{!site.Name}"/>
                    <apex:column value="{!site.Site_Placement_ID__c}"/>
                    <apex:column value="{!site.Site_Level_Max_Impressions__c}"/>
                   
                 </apex:pageBlockTable>
                 </apex:pageBlockSectionItem>
                
           </apex:pageBlockSection>
         </apex:pageblock>
                
</apex:form>
</apex:page>