[qml-plugin-calendar] Protect the calendar manager teardown more. Con…

…tributes to JB#51240

Avoid creating just to be deleted and avoid any chance of double
deletion.

src/calendarmanager.cpp

@@ -98,9 +98,10 @@ CalendarManager::CalendarManager()
     connect(mTimer, SIGNAL(timeout()), this, SLOT(timeout()));
 }
 
+static CalendarManager *managerInstance = nullptr;
+
 CalendarManager *CalendarManager::instance(bool createIfNeeded)
 {
-    static CalendarManager *managerInstance;
     if (!managerInstance && createIfNeeded)
         managerInstance = new CalendarManager;
 
@@ -111,6 +112,9 @@ CalendarManager::~CalendarManager()
 {
     mWorkerThread.quit();
     mWorkerThread.wait();
+    if (managerInstance == this) {
+        managerInstance = nullptr;
+    }
 }
 
 QList<CalendarData::Notebook> CalendarManager::notebooks()

src/plugin.cpp

@@ -97,7 +97,7 @@ class CalendarManagerReleaser: public QObject
         // will be destroyed via deleteLater when control returns to the event loop.
         // Deleting CalendarManager in NemoCalendarPlugin dtor is not an option
         // as it is called after the event loop is stopped.
-        delete CalendarManager::instance();
+        delete CalendarManager::instance(false);
     }
 };