Blocks with Custom Model do not render inside Bundle UI

When adding a custom block via add-on, the custom model is not rendered inside a bundle.

Steps to reproduce:

1. Create add-on with a custom block with custom model (e. G. using Block Wizard)

2. Grab a bundle and the new block

3. Put the new block into the bundle

Observed result:

The icon of the block is invisible once in the bundle

Expected result:

Icon renders correctly