Fix group ownership issue.

pcbnew/pcb_group.cpp

@@ -268,17 +268,22 @@ void PCB_GROUP::swapData( BOARD_ITEM* aImage )
 
     std::swap( *this, *image );
 
-    RunOnChildren(
+    // A group doesn't own its children (they're owned by the board), so undo doesn't do a
+    // deep clone when making an image.  However, it's still safest to update the parentGroup
+    // pointers of the group's children -- we just have to be careful to do it in the right
+    // order in case any of the children are shared (ie: image first, "this" second so that
+    // any shared children end up with "this").
+    image->RunOnChildren(
             [&]( BOARD_ITEM* child )
             {
-                child->SetParentGroup( this );
+                child->SetParentGroup( image );
             },
             RECURSE_MODE::NO_RECURSE );
 
-    image->RunOnChildren(
+    RunOnChildren(
             [&]( BOARD_ITEM* child )
             {
-                child->SetParentGroup( image );
+                child->SetParentGroup( this );
             },
             RECURSE_MODE::NO_RECURSE );
 }