Attempt to invoke virtual method...on a null object reference problem - azure

Question

I have a problem with my code, where sometimes I get a null object reference and sometimes not (on the same thing) and I cant understand whats the problem because as I said, one time its working fine but on the second time it shows me the null object reference message. What Im trying to do is to get the club of the logged trainer from my Azure table.
private MobileServiceTable<trainer> TrainerTable=null;
private MobileServiceClient mService=null;
private users cl=null;
private ProgressDialog prg;
private void Trainer_Club_String(final String username)
{
AsyncTask<Void,Void,Void> task = new AsyncTask<Void, Void, Void>()
{
#Override
protected Void doInBackground(Void... voids)
{
try
{
List<trainer> chosen_manager= TrainerTable.where().field("username").eq(username).execute().get(); \\the problem is in this line. Sometimes it tells me that the user is null and sometimes its working well
if(chosen_manager.size()>0)
{
trainer_club=chosen_manager.get(0).getClub().toString();
}
}
catch (Exception e)
{
final String message=e.getMessage().toString();
runOnUiThread(new Runnable()
{
#Override
public void run()
{
Toast.makeText(trainer_home_page.this, message, Toast.LENGTH_LONG).show();
}
});
}
return null;
}
}.execute();
}
ONCREATE:
cl=StaticObjects.GetClient();
trainer_username=cl.getUsername().toString();
Trainer_Club_String(trainer_username);