i am facing this error :Key cannot be null or empty and must be alphanumeric

here is the below code

public with sharing class SubmissionsSearchExt { public IBI_TM_Submissions__c SubmissionsVar {get;set;} public list<IBI_TM_Submissions__c> SubmissionsListVar {get;set;} public String sortField {get; set;} public String SortOrder {get; set;}

public SubmissionsSearchExt(ApexPages.StandardController controller) { if(cache.Session.contains('SubmissionsVar')) { cache.SessionPartition orgpart = cache.Session.getPartition('local.Searchcache'); SubmissionsVar= (IBI_TM_Submissions__c)orgpart.get('SubmissionsVar ');

    }
    else
    {
        SubmissionsVar = new IBI_TM_Submissions__c();
    }
//SubmissionsVar = new IBI_TM_Submissions__c();
SubmissionsListVar = new list <IBI_TM_Submissions__c>();
sortField = 'CreatedDate';
    sortOrder = 'desc'; 
    showlist1();
}

public void showlist1(){

   string stJob,stSub,stApp,stEndcli,stAcc,stName,stcon,stBen;
    string st = 'select id,Name,LastModifiedDate,CreatedDate,Job__c,Job_Title__c,Job_Status__c, Applicant_Name__c,Comment__c,Applicant__r.Last_Name__c,Applicant__r.First_Name__c,Submitted_By__c,Interview_Status__c,Contact__c,Applicant__c,Pay_Rate__c,Submission_Status__c,Applicant_Expected_Pay__c,Pay_Type__c,Submitted_on__c,Owner.Name,Account__c,Client_Billing_Rate__c,Bench__c,Priority__c,Comments__c,Internal_Interview_Date_Time__c,Client_Inteview_Date_Time__c,End_Client_Inteview_Date_Time__c from IBI_TM_Submissions__c';
    string sttemp = ''; 
    if(cache.Session.contains('SubmissionsVar'))
    {
         cache.Session.remove('SubmissionsVar');
    }
    cache.Session.put('local.Searchcache.SubmissionsVar', SubmissionsVar);
     if(SubmissionsVar.Job_Title__c != null  && SubmissionsVar.Job_Title__c != '')
       {
          stName = '%' + SubmissionsVar.Job_Title__c + '%' ;
          if(st.contains('where'))
          st = st + ' and Job_Title__c like :stName';
        else  
              st = st + ' where Job_Title__c like :stName';
              sttemp = 'test';
      }
     if(SubmissionsVar.Applicant_Name__c!= null  && SubmissionsVar.Applicant_Name__c!= '')
       {
          stApp = '%' + SubmissionsVar.Applicant_Name__c+ '%' ;
          if(st.contains('where'))
          st = st + ' and Applicant_Name__c like :stApp ';
        else  
              st = st + ' where Applicant_Name__c like :stApp ';
              sttemp = 'test';
      }
    if(SubmissionsVar.Account__c != null)
    {
        stAcc= SubmissionsVar.Account__c;
        if(st.contains('where'))
           st = st + ' and Account__c = :stAcc';
        else   
           st = st + ' where  Account__c = :stAcc';
        sttemp = 'test';
    } 
     if(SubmissionsVar.Submitted_By__c!= null && SubmissionsVar.Submitted_By__c!= '' )
    {
        stSub= SubmissionsVar.Submitted_by__c;
        if(sttemp != '' &&  sttemp != null)
           st = st + ' and  Submitted_by__c  = :stSub';
        else
           st = st + ' where  Submitted_by__c =  :stSub';
        sttemp = 'test';
    }
          st = st + ' order by ' + sortField  + ' ' + sortOrder + ' Limit 200';
              system.debug('test111' + st + SubmissionsListVar);       
              SubmissionsListVar = database.query(st);       
}
public PageReference save() {
upsert SubmissionsListVar ;
return ApexPages.currentPage();
}

public void doSort() { if( sortOrder == 'desc') sortOrder = 'asc'; else if( sortOrder == 'asc') sortOrder ='desc'; showlist1(); } } #Cache  #Platform Cache  #Salesforce Developer  #Error Message 

#Error  #Apex Class    #Apex  #Apex Code  #Salesforce Development  #Salesforce Platform #Salesforce Developer