The customer observed that after changing custom block definition, Landing Page where the block was used cannot be edited anymore.

Steps to reproduce:
1. Create a custom page block, ref: https://doc.ezplatform.com/en/2.5/guide/extending_page/#creating-custom-page-blocks.
2. Provide a simple template for your block.
3. Create a new Landing Page and put the new block inside.
4. Publish the Landing Page.
5. Change the block definition by removing one of the attributes.
6. Try to edit the created Landing Page.

Result:
In browser web tools console there is an error:

TypeError: Cannot read property 'constraints' of undefined

Expected result:
Modal for the custom block should be changed according to configuration meaning it should not contain missing attribute.