I am having one Set having data type as String and One List of type 'GroupMember'.

I want to type cast it . Tell me way to type cast it .

Set<String> mergelist=new Set<String>();

List<GroupMember>  newGroupMember = new List<GroupMember>(); 

I want to insert mergelist data into newGroupMember list.

I am getting follwing error when trying to create a new task and link it to incident.

"mytask13: execution of BeforeUpdatecaused by: System.NullPointerException: Attempt to de-reference a null object()"

I am trying update incident who hasn't assigned any template and attached to task.
Those incident who satisfy above condition I want their task's description to be updated and respected incident's template need to update.   

Following is the code I am using .

trigger mytask13 on BMCServiceDesk__Task__c(after update,before update)
{

        List<BMCServiceDesk__Incident__c> myinc{get;set;}
        
    for(BMCServiceDesk__Task__c myobject :Trigger.new)
    {
      List<BMCServiceDesk__Task__c> mytask = [SELECT id,name,BMCServiceDesk__FKIncident__c FROM BMCServiceDesk__task__c where BMCServiceDesk__FKIncident__c !='' and BMCServiceDesk__FKIncident__r.BMCServiceDesk__FKTemplate__c = ''];
               myobject.BMCServiceDesk__taskDescription__c = 'task description updated through trigger';
               myobject.BMCServiceDesk__FKIncident__r.BMCServiceDesk__FKTemplate__c= 'a3k7F0000004UNvQAM';
               
           
    }
}

I am getting follwing error when trying to create a new task and link it to incident.

"mytask13: execution of BeforeUpdatecaused by: System.NullPointerException: Attempt to de-reference a null object()"

I am trying update incident who hasn't assigned any template and attached to task.
Those incident who satisfy above condition I want their task's description to be updated and respected incident's template need to update.   

Following is the code I am using .

trigger mytask13 on BMCServiceDesk__Task__c(after update,before update)
{

        List<BMCServiceDesk__Incident__c> myinc{get;set;}
        
    for(BMCServiceDesk__Task__c myobject :Trigger.new)
    {
      List<BMCServiceDesk__Task__c> mytask = [SELECT id,name,BMCServiceDesk__FKIncident__c FROM BMCServiceDesk__task__c where BMCServiceDesk__FKIncident__c !='' and BMCServiceDesk__FKIncident__r.BMCServiceDesk__FKTemplate__c = ''];
               myobject.BMCServiceDesk__taskDescription__c = 'task description updated through trigger';
               myobject.BMCServiceDesk__FKIncident__r.BMCServiceDesk__FKTemplate__c= 'a3k7F0000004UNvQAM';
               
           
    }
}