How to reproduce:
1. Open users attached project
2. Inspect "Prefab", note that it has a "NewBehaviourScript" attached
3. Delete the 'NewBehaviourScript' script asset (from Project folder)
4. Drag 'Prefab' into Hierarchy/Scene

Expected result: "Prefab" text is still shown in a blue color, indicating that it is connected to a main "Prefab"
Actual result: "Prefab" text is shown in white, as it would be disconnected from a main "Prefab", clicking Apply or Revert throws a Null Reference Exception

Reproducible with - 2017.3.0a4, 2017.2.0f3, 2017.1.2f1, 5.6.3p4

Notes:
1. Clicking "Revert" or "Apply" on the "Prefab" instance causes a Null Reference Exception being thrown (the "Apply" one was not reproducible up until 2017.3.0b1 and 2017.2.0f3)
2. Even though the "Prefab" instance is shown as disconnected, new components and changes are still applied to a main (originating) Prefab

Fixed in - 2018.3.0b5