Reproduction steps:
1. Open the attached project "transition exception.zip" and undock the preview window (setup should look similar to the attached gif);
2. Close the editor and re-open it;
3. Open the animation controller "test controller";
4. Select the transition as shown in the GIF.

Expected results: Undocking the preview window should not trigger exceptions.
Actual results: the exceptions are generated in the console.

Reproduced on 5.5.3p3, 5.6.2p4, 2017.1.0f3, 2017.2.0b4, 2017.3.0a1.