v5.0
OSX
bl 2.78
I get this error when i try UV Manipulation > circle

Also your version behaves really different than the uv_tools version. Mesh influence is in the smooth tab. Whereas with uv_tools it also influenced when doing straighten action.
The straighten op doesnt work when wanting to straighten en line in the middle, seems to only work on outer edges.
Align UV shows this error

Sorry for posting images, but pasting code was not readable, no line breaks.
v5.0
OSX
bl 2.78
I get this error when i try UV Manipulation > circle

Also your version behaves really different than the uv_tools version. Mesh influence is in the smooth tab. Whereas with uv_tools it also influenced when doing straighten action.
The straighten op doesnt work when wanting to straighten en line in the middle, seems to only work on outer edges.
Align UV shows this error

Sorry for posting images, but pasting code was not readable, no line breaks.