How to reproduce:
1. Open the attached project
2. Open MainScene
3. Open Animator window and select cat_idle2_0 from the scene
4. Open PlayerControl script
5. In line 48 and 54 anim.SetFloat ("speed", 0.1f); change animation parameter's value from float to double (enough to delete f) and save the script
6. Go back to the project and notice that you have compiler error, change parameter back to float and save the script
7. Quickly go back to project and select one of the transitions. If you manage to select transition before it started compiling script you will get two errors:
"KeyNotFoundException: The given key was not present in the dictionary."
in 2017.3 one gets "NullReferenceException: Object reference not set to an instance of an object"

Reproducible: 5.6.3f1, 2017.1.1f1, 2017.2.0b9, 2017.3.0a6