platforminputcontexts: future-proof compose plugin

If this plugin is loaded at some later point during application
run-time, the focus object might be nullptr. We can avoid that by
using qApp->focusObject(), when m_focusObject==nullptr;

Task-number: QTBUG-74465
Change-Id: I0d82410ed557ea1a8fde28a1807f790854951cda
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/plugins/platforminputcontexts/compose/qcomposeplatforminputcontext.cpp

@@ -40,6 +40,7 @@
 
 #include <QtCore/QCoreApplication>
 #include <QtGui/QKeyEvent>
+#include <QtGui/QGuiApplication>
 
 #include <locale.h>
 
@@ -121,7 +122,14 @@ bool QComposeInputContext::filterEvent(const QEvent *event)
 
         QInputMethodEvent event;
         event.setCommitString(composedText);
-        QCoreApplication::sendEvent(m_focusObject, &event);
+
+        if (!m_focusObject && qApp)
+            m_focusObject = qApp->focusObject();
+
+        if (m_focusObject)
+            QCoreApplication::sendEvent(m_focusObject, &event);
+        else
+            qCWarning(lcXkbCompose, "no focus object");
 
         reset();
         return true;