Error from Companion: java.lang.RuntimeException: invalid syntax in eval form: :12:1: caught exception in inliner for # - java.lang.RuntimeException: no such class:

MIT App Inventor Help
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The MIT emulator package usually ends up with a fresh Android but a stale Companion each time it is updated.

It is simpler to separate the two, starting the Android emulator from your desktop OS, then running the Companion from the emulated Android. From there, check the running Companion version against the AI2 Help->Companion Information version.

(I personally use BlueStacks as my emulator of choice.)