Below is my VF code and the controller code.

When i hit the command button i get this white page and it says AN INTERNAL ERROR HAS OCCURRED.

Can't figure out, why this is happening, help please!!!!

<apex:page controller="workOrderController">
<apex:form >
<apex:pageBlock mode="maindetail" >
<apex:pageBlockSection columns="1" title="Number of Orders Involved">
<apex:actionRegion >
<apex:outputText value="Number of Orders Involved:"/>
<apex:selectList value="{!involved}" size="1">
<apex:selectOption itemValue="0" itemLabel="0"/>
<apex:selectOption itemValue="1" itemLabel="1"/>
<apex:selectOption itemValue="2" itemLabel="2"/>
<apex:selectOption itemValue="3" itemLabel="3"/>
<apex:actionSupport event="onchange" reRender="selection" status="status5" />
</apex:selectList>
</apex:actionRegion>
</apex:pageBlockSection>
<apex:outputPanel id="selection">
<apex:pageBlockSection id="section1" rendered="{!involved==1||involved==2||involved==3}">
<apex:outputLabel value="Order 1:" for="orderNumber1"/>
<apex:inputText id="orderNumber1" value="{!orderNumber1}"/>
</apex:pageBlockSection>
<apex:pageBlockSection id="section2" rendered="{!involved==2||involved==3}">
<apex:outputLabel value="Order 2:" for="orderNumber2"/>
<apex:inputText id="orderNumber2" value="{!orderNumber2}"/>
</apex:pageBlockSection>
<apex:pageBlockSection id="section3" rendered="{!involved==3}" >
<apex:outputLabel value="Order 3: " for="orderNumber3"/>
<apex:inputText id="orderNumber3" value="{!orderNumber3}"/>
</apex:pageBlockSection>
</apex:outputPanel>
<apex:pageBlockButtons >
<apex:commandButton action="{!displayCircuits}" value="Display Circuits"/>
</apex:pageBlockButtons>
</apex:pageBlock>
<apex:pageBlock title="Retrieved Circuit records">
<apex:dataList value="{!queryResult}" var="r">
<apex:outputText value="{!r.Id}" />
<apex:outputText value="{!r.Name}" />
</apex:dataList>
</apex:pageBlock>
</apex:form>
</apex:page>

public class workOrderController{

public Integer involved = 0;
public String orderNumber1{get{return orderNumber1;}set{orderNumber1 = value;}}
public String orderNumber2{get{return orderNumber2;}set{orderNumber2 = value;}}
public String orderNumber3{get{return orderNumber1;}set{orderNumber1 = value;}}
public List<Circuits__c> queryResult{get{return queryResult;} set{queryResult = value;}}
 
public Integer getInvolved(){

 return this.involved;
}

public void setInvolved(Integer involved){

    this.involved = involved;
}

public pageReference displayCircuits(){

String qryString = 'SELECT Id,Circuits__c.Name, Circuits__c.Circuit_ID__c'+
                     'FROM Circuits__c' +
                      'WHERE Order__r.Name =: ' +  orderNumber1 + 'or' +
                          'Order__r.Name =: ' + orderNumber2 + 'or'+
                          'Order__r.Name =:' + orderNumber3;
                 
queryResult = Database.query(qryString);                        
 return null;                        

}

}