Your using an invalid type as your blocks return or parameter type. As of Rush v1.2.5, you can only use the following types as parameters or return types:
Strings
Number types like int, float, double, etc.
java.util.Calendar
boolean
Classes of all the available AI2 components and other runtime classes, like Button, AndroidViewComponent, etc.
Some other AI2 types like YailList, YailObject and YailDictionary.
D:\Rush\web-x>java -version
java version "19" 2022-09-20
Java(TM) SE Runtime Environment (build 19+36-2238)
Java HotSpot(TM) 64-Bit Server VM (build 19+36-2238, mixed mode, sharing)
Error
┌ Checking project files
│ info Checking metadata file (rush.yml)
│ info Checking AndroidManifest.xml file
└ done
┌ Compiling sources
│ info Picked 1 source file
└ done
┌ Processing the extension
│ info Linking extension assets
│ info Generating DEX bytecode
│ Compilation failed with an internal error.
│ java.lang.IllegalArgumentException: Unsupported class file major version 63
│ at com.android.tools.r8.A.a.a.f.<init>(:10)
│ at com.android.tools.r8.A.a.a.f.<init>(:2)
│ at com.android.tools.r8.A.a.a.f.<init>(:1)
│ at com.android.tools.r8.A.a.a.f.<init>(:107)
│ at com.android.tools.r8.graph.x0.a(:24)
│ at com.android.tools.r8.dex.a$a.a(:12)
│ at java.base/java.util.concurrent.ForkJoinTask$AdaptedCallable.exec(ForkJoinTask.java:1456)
│ at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
│ at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1311)
│ at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1840)
│ at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1806)
│ at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
└ failed
• Build failed [4s 122ms] []