Check for qApp before using it in ibus plugin

We might call updatePreeditText from the QCoreApplication dtor by
running an event loop from inside a "post routine" added with
qAddPostRoutine().

Task-number: QTBUG-60000
Change-Id: I04c08fe36bfa63ac345a06e50952d2ec83a78ac0
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforminputcontexts/ibus/qibusplatforminputcontext.cpp

@@ -267,6 +267,9 @@ void QIBusPlatformInputContext::commitText(const QDBusVariant &text)
 
 void QIBusPlatformInputContext::updatePreeditText(const QDBusVariant &text, uint cursorPos, bool visible)
 {
+    if (!qApp)
+        return;
+
     QObject *input = qApp->focusObject();
     if (!input)
         return;