[Autocomplete] RuntimeError on hitting Enter after inserting a value when multiple

[Joostluijben]

When hitting enter after adding a value to a multiple select field, the element causes a RunTimeError

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The Autocomplete element causes a runtime error when adding a value to a multiple select field and afterward hitting enter. I think the onChange is triggered and because there's no value to change it gives an undefined error.

Expected Behavior 🤔

The element shouldn't respond to the enter key when there's no input

Steps to Reproduce 🕹

Steps:
Live example: https://codesandbox.io/s/kyp0d

1. In one of the select elements, select a value
2. Hit enter
3. The app should crash

Context 🔦

My company uses this for auto completion of some values. When the users hits enter after entering a value they might think the app doesn't work.

Your Environment 🌎

Tech	Version
Material-UI	v4.5.2
React	16.9.0
Browser	Firefox 70.0.1
TypeScript	3.3.0
Material-UI/lab	4.0.0-alpha.31

[oliviertassinari]

This is a duplicate of #18221. Thank you for the report. The issue will be solved in the next release: v4.0.0-alpha.32.