buildTarget/cleanCache throws an error

[ckipp01]

When using Mill as a BSP server I noticed that I'm getting an internal error when I try to do a clean compile. For example in the logs I see

[Trace - 02:09:53 pm] Sending request 'buildTarget/cleanCache - (24)'
Params: {
  "targets": [
    {
      "uri": "file:///Users/ckipp/Documents/scala-workspace/mill-plugins/mill-giter8/plugin/0.10"
    },
    {
      "uri": "file:///Users/ckipp/Documents/scala-workspace/mill-plugins/mill-giter8/plugin/0.11"
    },
    {
      "uri": "file:///Users/ckipp/Documents/scala-workspace/mill-plugins/mill-giter8/mill-build"
    }
  ]
}

...

[Trace - 02:09:53 pm] Received response 'buildTarget/cleanCache - (24)' in 4ms
Result: null
Error: {
  "code": -32603,
  "message": "Internal error.",
  "data": "java.util.concurrent.CompletionException: scala.MatchError:  (of class millbuild.build$)\n\tat java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:332)\n\tat java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:347)\n\tat java.base/java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:708)\n\tat java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)\n\tat java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2162)\n\tat mill.bsp.worker.MillBuildServer.$anonfun$completable$1(MillBuildServer.scala:708)\n\tat mill.bsp.worker.MillBuildServer.$anonfun$completable$1$adapted(MillBuildServer.scala:696)\n\tat scala.concurrent.impl.Promise$Transformation.run(Promise.scala:484)\n\tat java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)\n\tat java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)\n\tat java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)\n\tat java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)\n\tat java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)\n\tat java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)\nCaused by: scala.MatchError:  (of class millbuild.build$)\n\tat mill.bsp.worker.MillBuildServer.$anonfun$buildTargetCleanCache$2(MillBuildServer.scala:595)\n\tat scala.collection.immutable.List.map(List.scala:246)\n\tat scala.collection.immutable.List.map(List.scala:79)\n\tat mill.bsp.worker.MillBuildServer.$anonfun$buildTargetCleanCache$1(MillBuildServer.scala:595)\n\tat mill.bsp.worker.MillBuildServer.$anonfun$completable$1(MillBuildServer.scala:699)\n\t... 8 more\n"
}

This then causes the compilation to be cancelled in the clean compile command.

To reproduce

1. Make a simple hello world project
2. Open with Metals and make sure everything is imported while using Mill as your build server
3. Create a .metals/bsp.trace.json file
4. Restart the build server
5. Trigger a Metals Clean Compile command
6. See error above in logs