If there are no initialized flex bones in the armature, clicking "Bake" will initialize the selected bones, set the range to the current scene range and then bake.
If the armature has no initialized flex bones, clicking on the bake button will initialize the selected bones, and set the bake range to the scene frame range before baking.
Also a few changes due to blender 2.8 api changes.
2.79 version with the same updates as the v0.3.3 Blender 2.80 version
Stable version for Blender 2.8 (unless the API changes!) Includes a fix to handle the display scaling of bendy bones