Fix crash when 'use popup' is selected for time based datetime widgets

(cherry-picked from 63cc762)

Author: nyalldawson

src/gui/editorwidgets/qgsdatetimeeditwrapper.cpp

@@ -66,7 +66,7 @@ void QgsDateTimeEditWrapper::initWidget( QWidget *editor )
 
   const bool calendar = config( "calendar_popup", false ).toBool();
   mQDateTimeEdit->setCalendarPopup( calendar );
-  if ( calendar )
+  if ( calendar && mQDateTimeEdit->calendarWidget() )
   {
     // highlight today's date
     QTextCharFormat todayFormat;

src/gui/editorwidgets/qgsdatetimesearchwidgetwrapper.cpp

@@ -156,7 +156,7 @@ void QgsDateTimeSearchWidgetWrapper::initWidget( QWidget* editor )
 
     const bool calendar = config( "calendar_popup", false ).toBool();
     mDateTimeEdit->setCalendarPopup( calendar );
-    if ( calendar )
+    if ( calendar && mDateTimeEdit->calendarWidget() )
     {
       // highlight today's date
       QTextCharFormat todayFormat;