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' event to one group and a 'Receive' event to the other group
5. Select the group with the 'Send' event and set its 'Receive' target to the other group
6. Right-click the 'Send Level' label and expose this parameter
7. Set the 'Receive' target to 'None'

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

Reproducible with: 2021.1.8f1, 2021.2.0a16, 2021.2.0a17
Not reproducible with: 2018.4.35f1, 2019.4.26f1, 2020.3.9f1, 2021.1.7f1, 2021.2.0a1, 2021.2.0a15

Notes:
2018.4 and 2019.4 seem to throw the assertions after step 6/step7(possibly Case 1178055), but the exposed parameters drop-down is still functioning
An Editor restart is required to fix the drop-down, but this only works when exposed parameter is "unhidden" again(in this case by setting the receive target)