Hi ,
Ia m getting the following error while runnung the code it is @line 26 in my code i have marked the line 26 please help me on this to solve the error
Visualforce Error

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System.NullPointerException: Argument 1 cannot be null
Class.FAQCu.getSearch: line 26, column 1 )

In the above code the line 26 is marked below it is in getSearch method below is the code where i have hilighted line 26 in the code

public class FAQCu {
public String selectedValue {get;set;}

public List<SelectOption> getUserType()
{
  List<SelectOption> options = new List<SelectOption>(); 
   Schema.DescribeFieldResult fieldResult =FAQ__c.Users__c.getDescribe();
   List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
     // options.add(new SelectOption('All','All'));
   for( Schema.PicklistEntry f : ple)
   {
      options.add(new SelectOption(f.getLabel(), f.getValue()));
   } 
   return options;
}
public List<FAQ__c> getSearch(){
        System.debug('====selectedValue===='+selectedValue); // You will get the Selected Value here
     
        List<FAQ__c> temp = new List<FAQ__c>();
        String queryString;
   if(selectedValue=='AllUsers')
   queryString='SELECT Users__c,Question__c,Answer__cfrom FAQ__c WHERE Users__c =AllUsers';
   else if(selectedValue=='InternalUsers')
   queryString='SELECT Users__c,Question__c,Answer__c from FAQ__c WHERE Users__c =InternalUsers';
   else if(selectedValue=='ExternalUsers')
queryString='SELECT Users__c,Question__c,Answer__c from FAQ__c WHERE Users__c =ExternalUsers';
26line//   temp = Database.query(queryString);

      return temp;
      }
    }

VF code 
<apex:page controller="FAQCu" id="theRepeat">
  <apex:form >
    <apex:selectList id="users" value="{!selectedValue}" size="1" required="true">
        <apex:selectOptions value="{!userType}"/>
        <apex:actionSupport event="onchange" rerender="questions"/>
    </apex:selectList>
    <apex:dataTable value="{!search}" var="faq" id="questions">
        <apex:column >
            <apex:outputText value="{!faq.Question__c}"/>
        </apex:column>
        <apex:column >
            <apex:outputText value="{!faq.Answer__c}"/>
        </apex:column>
    </apex:dataTable>

</apex:form>

</apex:page>

Thanks
Veeraiah