How to reproduce:
1. Create a new Unity project
2. Create an AudioMixer asset and open it in AudioMixer window
3. Add 2 new child groups of the Master group
4. Add a 'Send' effect to group A and a 'Receive' effect to group B
5. Select group A and set its 'Receive' target to group B
6. Right-click the 'Send Level' label and expose this parameter

Then one of the following:

7a. Delete the send effect on group A
7b. Delete the receive effect on group B
7c. Delete group B

Expected result: no errors are thrown, Exposed Parameters drop-down works
Actual result: an assertion failed error is thrown, Exposed Parameters drop-down no longer works and throws a NullReferenceException

Note: There could be other workflows involving exposed send levels that also lead to this assertion.