The already assigned content doesn't appear in the list of available contents. If only one content is available to the page (the already assigned one), the form complains about saving a page without an assigned content. The problem is the order of execution in the PageEditForm. The available contents are read in the readData method. The result is correctly stored. But the same method of the PageAddForm is called afterwards and overwrites the result.
The already assigned content doesn't appear in the list of available contents. If only one content is available to the page (the already assigned one), the form complains about saving a page without an assigned content. The problem is the order of execution in the PageEditForm. The available contents are read in the readData method. The result is correctly stored. But the same method of the PageAddForm is called afterwards and overwrites the result.