How to reproduce:
1. Create a new Unity Project
2. Go to Edit -> Project Settings -> Input Manager
3. Open the Axes array, right-click on any Axis, and press "Delete Array Element"

Expected result: The Axis is deleted from the Array
Actual result: The Axis is not deleted and an error is thrown in the console

Reproducible with: 2021.1.0a1, 2021.1.0a10
Not reproducible with: 2018.4.30f1, 2019.4.16f1, 2020.1.17f1, 2020.2.0a21, 2020.2.0f1

Note:
- Input Manager is named Input in 2018.4.30f1
- Error thrown: KeyNotFoundException: The given key was not present in the dictionary.