Fix a race condition on startup that would cause a crash if the album cover loader thread took too long to start (happened a lot on Windows)

src/widgets/nowplayingwidget.cpp

@@ -164,6 +164,10 @@ void NowPlayingWidget::CoverLoaderInitialised() {
 }
 
 void NowPlayingWidget::UpdateHeight(AlbumCoverLoader* loader) {
+  if (!loader) {
+    return;
+  }
+
   switch (mode_) {
   case SmallSongDetails:
     cover_height_ = small_ideal_height_;