Do not paint WMS preview images. Because it crashes.

Fix #16803

src/providers/wms/qgswmsprovider.cpp

@@ -615,11 +615,16 @@ static void _drawDebugRect( QPainter& p, const QRectF& rect, const QColor& color
 
 QImage *QgsWmsProvider::draw( QgsRectangle const & viewExtent, int pixelWidth, int pixelHeight, QgsRasterBlockFeedback* feedback )
 {
-  QgsDebugMsg( "Entering." );
+  QImage* image = new QImage( pixelWidth, pixelHeight, QImage::Format_ARGB32 );
+
+  if ( QgsApplication::instance()->thread() == QThread::currentThread() )
+  {
+    QgsDebugMsg( "Trying to draw a WMS image on the main thread. Stop it!" );
+    return image;
+  }
 
   // compose the URL query string for the WMS server.
 
-  QImage* image = new QImage( pixelWidth, pixelHeight, QImage::Format_ARGB32 );
   image->fill( 0 );
 
   if ( !mSettings.mTiled && mSettings.mMaxWidth == 0 && mSettings.mMaxHeight == 0 )