I tried to reproduce this in 4.12.5 (both the binary version as well as when built from source code), and was unable to see the error that you mentioned after adding a new Toolbar button plugin. Would you be able to elaborate on the steps you are following when you add the plugin?
We have not heard back from you for a while. Do you still need help with this issue? I will be marking this post as resolved for internal tracking purposes, but please feel free to add a comment and re-open the post if you need any additional assistance.