Debugger: Fix potential VariablesView crash.

- In some contexts, VariablesView doesn't have an associated thread
  and stack frame, which would lead to a potential crash when resolving
  expression nodes.

src/apps/debugger/user_interface/gui/team_window/VariablesView.cpp

@@ -2560,8 +2560,8 @@ VariablesView::_RequestNodeValue(ModelNode* node)
 	containerLocker.Unlock();
 
 	// request resolution of the value
-	fListener->ValueNodeValueRequested(fStackFrame->GetCpuState(), container,
-		valueNode);
+	fListener->ValueNodeValueRequested(fStackFrame != NULL
+			? fStackFrame->GetCpuState() : NULL, container, valueNode);
 }