Data corrupted message when moving a page to an unsaved structure element in gallery

[solth]

Describe the bug
When moving a page via drag'n'drop in the gallery from "unstructured media" to a new structure element which has not been saved, yet, a popup dialog about corrupted data appears (see screenshot below). The same does not happen when moving a page to an unsaved structure element in the structure tree, however. Saving the structure before the drag'n'drop action prevents the error.
Additionally, the popup dialog cannot be closed by clicking on the "Ok" button. The metadata editor page needs to be reloaded to remove the dialog.

To Reproduce
Steps to reproduce the behavior:

1. Open process containing at least one page in the metadata editor
2. Create a new structure element
3. Do not save the process after creating the element!
4. Try to move the page via drag'n'drop to the newly created, unsaved structure element in the gallery
5. See error

Expected behavior
It should be possible to move pages to an unsaved structure element in the gallery without an error. In situations where it is warranted that the dialog appears, it should be possible to close it with the "Ok" button.

Screenshots

Release
current master, 3.7.0-SNAPSHOT

[andre-hohmann]

@solth : thanks a lot for this!
In SLUB Dresden this occurs sometimes with loss of data (structural elements, metadata, ...). Thus, this is for us an issue of high priority.
I just had problems to describe the possible cause for the behavior. I would never had thought, that it is because an unsaved structure element.

Here is the error message in German: