Adding key to a Rect Transform property in the Animation Window generates a NullReferenceException error.

repo:
-create a UI>Image in the scene view
-select Canvas GO
-create a clip in the AW
-select Image GO
-scrub AW timeline to 1.0 sec (frame 60)
-animate the Rect Transform > Pos X, by changing the value in the Inspector.
-move the current time to 0.5 sec (frame 30)
-click the Add Key button... error.

package repro:
-import package
-open rectBug scene
-open AW
-select Image GO under Canvas
-add key to existing animated property in AW... error.

This is a regression
repro in...
Version 5.5.0f3 (38b4efef76f0)
Thu, 24 Nov 2016 13:54:18 GMT
Branch: 5.5/release
... and 5.6

not repro in...
Version 5.4.3f1 (01f4c123905a)
Wed, 09 Nov 2016 12:07:06 GMT
Branch: 5.4/release