Fix error when no valid memory reference

Author: puremourning

python3/vimspector/variables.py

@@ -675,7 +675,7 @@ def GetMemoryReference( self ):
     # Get a memoryReference for use in a ReadMemory request
     variable, _ = self._GetVariable( None, None )
     if variable is None:
-      return None
+      return None, None
 
     # TODO: Return the connection too!
     return variable.connection, variable.MemoryReference()