Crash when minimizing the animation preview window.

Steps to reproduce
- Open the attached project with 2019.2.0a12.
- In the projects window select Assets/ReleaseQA/Animations/Release QA - Walk Controller.blend
- Double click to open the Animator window.
- Double click on the Locomotion node to access the blend nodes.
- Select the Speed node.
- In the Inspector, press play in the animation preview window.
- Right-click the preview window title "Speed" to undock the preview window.
- Right-click preview window title again to dock the window.
Notice Issue >> Crash

Working: 2019.1.0f2, 2019.2.0a6, 2019.2.0a9, 2019.2.0a11
Broken: 2019.2.0a13, 2019.2.0a12