Fix crash when grass provider is not available

Fixes #21604

Author: nyalldawson

src/core/qgsmaplayerproxymodel.cpp

@@ -117,7 +117,7 @@ bool QgsMapLayerProxyModel::filterAcceptsRow( int source_row, const QModelIndex
   if ( mExceptList.contains( layer ) )
     return false;
 
-  if ( mExcludedProviders.contains( layer->dataProvider()->name() ) )
+  if ( layer->dataProvider() && mExcludedProviders.contains( layer->dataProvider()->name() ) )
     return false;
 
   if ( mFilters.testFlag( WritableLayer ) && layer->readOnly() )