[nemo-qml-plugin-calendar] Avoid unnecessary instantiation of calenda…

…r manager. Contributes to JB#43886

Avoids recreating NemoCalendarManager in invitation query dtor,
which can happen if the query instance was never really used for anything.

src/calendarinvitationquery.cpp

@@ -44,7 +44,10 @@ NemoCalendarInvitationQuery::NemoCalendarInvitationQuery()
 
 NemoCalendarInvitationQuery::~NemoCalendarInvitationQuery()
 {
-    NemoCalendarManager::instance()->unRegisterInvitationQuery(this);
+    NemoCalendarManager *manager = NemoCalendarManager::instance(false);
+    if (manager) {
+        manager->unRegisterInvitationQuery(this);
+    }
 }
 
 QString NemoCalendarInvitationQuery::notebookUid() const

src/calendarmanager.cpp

@@ -98,10 +98,10 @@ NemoCalendarManager::NemoCalendarManager() :
     connect(mTimer, SIGNAL(timeout()), this, SLOT(timeout()));
 }
 
-NemoCalendarManager *NemoCalendarManager::instance()
+NemoCalendarManager *NemoCalendarManager::instance(bool createIfNeeded)
 {
     static NemoCalendarManager *managerInstance;
-    if (!managerInstance)
+    if (!managerInstance && createIfNeeded)
         managerInstance = new NemoCalendarManager;
 
     return managerInstance;

src/calendarmanager.h

@@ -59,7 +59,7 @@ class NemoCalendarManager : public QObject
     NemoCalendarManager();
 
 public:
-    static NemoCalendarManager *instance();
+    static NemoCalendarManager *instance(bool createIfNeeded = true);
     ~NemoCalendarManager();
 
     NemoCalendarEvent* eventObject(const QString &eventUid, const KDateTime &recurrenceId);