Fix crash on closing

Test Plan: Start in windowed mode, close. Doesn't crash anymore.

Reviewers: #plasma, #kwin, mart

Reviewed By: #plasma, #kwin, mart

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19919

virtualkeyboard.cpp

@@ -243,7 +243,7 @@ void VirtualKeyboard::updateInputPanelState()
 
     auto t = waylandServer()->seat()->focusedTextInput();
 
-    if (!t) {
+    if (!t || !m_inputWindow) {
         return;
     }