How to reproduce:
1. Open the attached 'project-1259577.zip' project
2. Enter/Exit Play mode (no error should be thrown)
3. Open the 'Controls' asset
4. Delete the 'TestAction' action
5. Press the 'Save Asset' button
6. Enter Play mode
7. Observe the Console window

Expected result: no error should be thrown
Actual result: an error is thrown

Reproducible with: 2019.4.3f1, 2020.1.0b15, 2020.2.0a17 (preview - 1.0.0, 1.0.0)
Could not test with: 2018.4.24f1 (and lower Input System package versions) due to incompatibilities

Thrown error:
Cannot find action 'TestMap/TestAction[/Keyboard/space]' with ID '94026b3b-de17-477b-ace8-1634fc257995' in 'Controls (UnityEngine.InputSystem.InputActionAsset)
UnityEngine.InputSystem.PlayerInput:OnEnable() (at Library/PackageCache/com.unity.inputsystem@1.0.0/InputSystem/Plugins/PlayerInput/PlayerInput.cs:1564)

Note: opening the Inspector window of the game object with the 'Player Input' asset that the Input Map is referencing, solves the issue and error is no longer thrown