Make sure that agenda is not null

CCBUG: 384324

src/agenda/timelabels.cpp

@@ -174,6 +174,9 @@ void TimeLabels::setAgenda(Agenda *agenda)
 /** This is called in response to repaint() */
 void TimeLabels::paintEvent(QPaintEvent *)
 {
+    if (!mAgenda) {
+        return;
+    }
     QPainter p(this);
 
     const int ch = height();

src/agenda/timelabels.h

@@ -87,10 +87,10 @@ private:
     int mRows;
     double mCellHeight;
     int mMiniWidth;
-    Agenda *mAgenda;
-    TimeLabelsZone *mTimeLabelsZone;
+    Agenda *mAgenda = nullptr;
+    TimeLabelsZone *mTimeLabelsZone = nullptr;
 
-    QFrame *mMousePos;  // shows a marker for the current mouse position in y direction
+    QFrame *mMousePos = nullptr;  // shows a marker for the current mouse position in y direction
 };
 
 }