Qt: Fix intermittent crash on exit (#6612)

* Fix null pointer deref on exit

* Fix from review

src/citra_qt/bootmanager.cpp

@@ -587,7 +587,9 @@ bool GRenderWindow::event(QEvent* event) {
 
 void GRenderWindow::focusOutEvent(QFocusEvent* event) {
     QWidget::focusOutEvent(event);
-    InputCommon::GetKeyboard()->ReleaseAllKeys();
+    if (auto* keyboard = InputCommon::GetKeyboard(); keyboard) {
+        keyboard->ReleaseAllKeys();
+    }
     has_focus = false;
 }