Don't crash when canceling graphics import

Grouped graphics need to have their group removed from the preview
before deleting the elements to avoid reading freed memory when
redrawing

pcbnew/tools/drawing_tool.cpp

@@ -1602,6 +1602,12 @@ int DRAWING_TOOL::PlaceImportedGraphics( const TOOL_EVENT& aEvent )
         {
             m_toolMgr->RunAction( PCB_ACTIONS::selectionClear, true );
 
+            if( group )
+            {
+                preview.Remove( group );
+                group->RemoveAll();
+            }
+
             for( BOARD_ITEM* item : newItems )
                 delete item;