Error on selection of Animator window tab

1. What happened
I'm getting 2 error messages when selecting the Animator window tab, when character is in the scene using a Controller...
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Unable to load flow connection texture.png
-and-
Unable to load flow selected connection texture.png
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This does not happen in 5.1

2. How we can reproduce it using the example you attached
Open Project
Open ErrorBug scene
-or-
Drop mangagirl into the scene
Drop Idle clip onto mangagirl to create Controller with clip
(if you don't already get the error)
Click Play in the Editor and select the Animator window tab

See attached video...