Now we have an inconclusive test....
I fired up the emulator to test the same project. Of five attempts it showed the runtime error once, the other four times it just crashed back to the emulator main screen.
I removed the extension and tried again, the project again would not load.
So thumbs down for the emulator all round from me at the moment.
Given that you do not have a real device, (you've been working on AppInventor for 6 years, and no real device ?) I suggest you try Genymotion, which provides an Android 10 emulator. This runs well on Linux, you need to use the Emulator.apk for x86, and also use tick the Legacy box. Ensure you set the virtual device to "bridged" in the settings so that the emulator is running on the same network. Oh, you will probably need @ 1.5gb of RAM.
Hi
Thanks for your help.
I think we are very close to isolate the problem.
In fact I do install the Genymotion. In the process, I must update the adb executable on the GNU/Linux side, install the ARM_Translation on the emulated Android device and reconfigure my VirtuallBox network's) but, at the end, all worked really fine. I will explore this new (for me) environment in the near future.
The bad news: I do receive exactly the same error.
I know it's not easy, but belive me I don´t use cel phone 
The good news:
I tried using my wife's phone and, like we expected, all works fine. So the problem is related to the emulator (all of them, the AppInventor bundled, the Android SDK bundled and the Genymotion)
After connect the real device I do try again with the emulated device and all worked!
In brief:
The problem "dissapear" after we establish a connection with a real device. In order to make it fail you must start a fresh work session (close your AI2 session, stop the aiStarter, emulator) and try to establish a connection with the emulated device without connect your real device. In my environment this cause a error 100% of the times.
Now I suspect that Rendezvous server/connection can be related to this error
¿What do you think?
Gus
Companion 2.63 on Huawei P10 Lite Android 8 wired via USB. APK works ok on the same phone.
Error from Companion: java.lang.RuntimeException: invalid syntax in eval form: :2300:1: caught exception in inliner for # - java.lang.RuntimeException: no such class: com.sunny.CustomWebView.CustomWebView ..........
Hello, I have the same problem (the first message in this conversation). I'm using Windows 11 + Chrome. I have just installed the AppInventor 2 software, so it is updated. I'm a teacher and I can't use mobile devices with my students. So, I really need this emulator working.
Could anyone help me?
Your errors are consistent with an outdated Companion, which loads separately from the emulator.
For a more current emulator and Companion, see the first link in
The newer emulator comes preloaded with a more current Companion.
Thank you! It worked 
Hello, I also teach with AppInventor and have the same issue as vita_h. ABG user told to use the "current emulator ... see the first link". Do you mean the Emulator Open Beta (MIT_App_Inventor_Tools_30.263.0_win_setup64.exe)?
I tried it and it worked, but this file is much larger than version 2.3.0. Also the installation crashed when it tried to install Intel Hardware Accelerated Execution Manager due to missing system requirements. Do I have to install this version now on any classroom pc anyway for using App Inventor?
Thank you
I have a Windows 11 laptop like that.
I use an Android phone when testing on that laptop, to avoid the aggravation of emulation.
Here's what I have heard about different emulators ...
What are the technical specs for the computer where the emulator is crashing? The Intel HAXM support is only for 64-bit Intel processors so that might be a source of your issue.
Hello, I have the same problem (the first message in this conversation). I'm using Windows 10 + Chrome. I have just installed the AppInventor 2 software, so it is updated.
Could anyone help me?
Hello, I have the same problem with any extension I install (Error from Companion: java.lang.RuntimeException: invalid syntax in eval form: :63:1: caught exception in inliner for # - java.lang.RuntimeException: no such class:). I'm using Windows 10 + Chrome. I have just installed the AppInventor 2 software, so it is updated.
Could anyone help me?
The class name that comes afterward is significant. If you see something along the lines of "com.google.appinventor.components" then you likely installed the older emulator package version 2.x and need to use the newer version or upgrade your companion app.
If it's another class name, then I have to say that unfortunately we don't support extensions in the emulator package at this time.