Notebook version: 5.3.1

How to reproduce:

1. Set an empty cell's type to 'Raw NBConvert' from the menu
2. Show raw cell format with 'View -> Cell Toolbar -> Raw Cell Format'
3. Select 'custom' in the cell toolbar

This makes a black overlay across the entire notebook, like a modal is going to open. But a modal never opens, and the notebook UI is inaccessible. I need to refresh page to fix this.