The Transmutr tool palette always opens when SketchUp starts up. It should only do that if the user left the palette open in the last session.
It should be using toolbar.restore instead of toolbar.show. Restore won’t show the toolbar if the user closed it in the last SketchUp session.
If all my plugins did this, the screen would be covered with palettes. I’m using SketchUp 20.0.362 on macOS High Sierra 10.13.6 and Mojave 10.14.6. This is quite a common problem with extensions on any SketchUp version on the Mac.