Android SQLite database...nullpointerException error while accesing table

public Cursor getPro()  
{  
sql=dbhelper.getReadableDatabase();  
return db.query("pro", new String[] {jid,username,password},null,null,null,null,null);  

}

its my getpro method...and here i'm accessing it.

                 c=db.getPro();
                 if(c != null $$ c.moveToFirst())
                 {
                     Toast.makeText(getApplicationContext(), "True", Toast.LENGTH_LONG).show();
                 }
                 db.close();

i want to check whether table has any data or not but app crashes with nullpointerException