Avoid QObject::connect null warning

(cherry picked from commit a8ecc68)

Author: nyalldawson

src/gui/qgsmapcanvas.cpp

@@ -172,9 +172,11 @@ QgsMapCanvas::QgsMapCanvas( QWidget *parent )
   moveCanvasContents( true );
 
   // keep device pixel ratio up to date on screen or resolution change
-  connect( window()->windowHandle(), &QWindow::screenChanged, this, [ = ]( QScreen * ) {mSettings.setDevicePixelRatio( devicePixelRatio() );} );
   if ( window()->windowHandle() )
+  {
+    connect( window()->windowHandle(), &QWindow::screenChanged, this, [ = ]( QScreen * ) {mSettings.setDevicePixelRatio( devicePixelRatio() );} );
     connect( window()->windowHandle()->screen(), &QScreen::physicalDotsPerInchChanged, this, [ = ]( qreal ) {mSettings.setDevicePixelRatio( devicePixelRatio() );} );
+  }
 
   connect( &mMapUpdateTimer, &QTimer::timeout, this, &QgsMapCanvas::mapUpdateTimeout );
   mMapUpdateTimer.setInterval( 250 );