Using Reusable Blocks in the customizer / widgets

When creating a new Reusable block and wanting to use it in a widget, when adding the block you are presented with an error.
> This block has encountered an error and cannot be previewed.

#### Steps to reproduce:
1. Ensure you have a Reusable Block to work with.
2. Navigate to Appearance -> Widgets
3. Add a new Reusable Block to one of the widgets.
4. When you hover the block to make your selection, you are present with the error mentioned above.

#### Describe what you expected to see:
I would have expected that a Reusable Block can be used in a widget area without any errors.

#### Acceptance criteria:
- [ ] Adding a Reusable Block to the widget area without seeing any errors.