QmakeProjectManager: Add a soft assert

Task-number: QTCREATORBUG-24683
Change-Id: I6c4780a3200eff7c371dd90bb0b441ce84d4b9cb
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmakeprojectmanager/qmakeparsernodes.cpp

@@ -233,6 +233,7 @@ QmakePriFile::~QmakePriFile()
 
 void QmakePriFile::scheduleUpdate()
 {
+    QTC_ASSERT(m_buildSystem, return);
     QtSupport::ProFileCacheManager::instance()->discardFile(
                 filePath().toString(), m_buildSystem->qmakeVfs());
     m_qmakeProFile->scheduleUpdate(QmakeProFile::ParseLater);