[MdAutocomplete] JS error when using a mix of typing and arrow keys

[simoneb]

Steps to reproduce

• click on the autocomplete input to open the autocomplete list
• type the letter a
• hit backspace to delete the letter a from the input
• hit ESC
• hit Arrow Down key

Which browser?

Chrome Version 69.0.3497.42 (Official Build) beta (64-bit) on Windows

What is expected?

• no JS error

What is actually happening?

• a JS error is reported: Cannot set property 'highlighted' of null

Reproduction Link

https://codesandbox.io/s/q4nml15y6w