The Add Component Menu will not open if a field in the inspector is focused (highlighted).

Reproduction Steps:
1. Create a new project
2. Select any gameobject with a component attached (or you could just use the transform on a blank gameobject).
3. Click on the name of a variable in the inspector, the field should highlight as blue
4. Press Ctrl+Shift+A - Notice the Add Component menu will not appear
6. Click a different gameobject in the hierarchy
7. Press Ctrl+Shift+A - Notice the Add Component STILL does not appear(!)
8. Select a component field again, then click underneath the "Add Component" button (in a blank area)
9. Ctrl+Shift+A will now work again

Regression introduced 2019.1.0a10, does not reproduce 2019.1.0a9

Although a bit fiddly to reproduce, once you have it, it is very confusing and difficult to fix intuitively. It even persists across changing scenes- so wouldn't be surprised if a lot of users resort to restarting the editor to fix it.