Reproduction steps:
1. Open the attached project "FBXBugReport.zip";
2. Go to the Assets/Models/JALC2_Ellie (this is an FBX file);
3. Click on the small arrow sign that expands the FBX file;
4. Click on the animation titled `Take 001`;
5. In the Inspector, click on the Edit button;
6. In the new Inspector tab, click on the Rig (the middle tab);
7. Change the Root Node to `Bubble_hips` and click Apply;

Expected results: The Root Node is changed correctly;
Actual results: The Editor crashes after applying.

Reproducible on: 5.5.0f1, 5.5.0p2, 5.6.0b3.
Not reproducible: 5.5.0a3, 5.4.3f1, 5.4.0b18, 5.3.6p6.
Regression firstly observed: 5.5.0f1.

Reproducible both on Windows (10 and 8) and on MacoOSX 10.12.