Hello Hmcll,
After looking over your crash report, it seems that this is a known issue (UE-23217) and that it has been reported to the developers for further consideration. I have provided a link to the original issue and I have also provided a workaround that has been met with success. Please visit the link provided for future updates on this issue. I hope that this information helps.
Link: Assertion failed: IsValid() File:D:\BuildFarm - UI - Epic Developer Community Forums
Workaround:
This issue in caused by closing the animation panel within the widget blueprints. Could you try resetting your layout?
Make it a great day