Check if treeDeleter is actually async before casting it. Fixes #13240.

This code assumed that `registerSpawnStrategies` would be always be called before `afterCommand`, but that's not the case if the build was interrupted or failed before the execution phase.

RELNOTES: None.
PiperOrigin-RevId: 379937925

Author: larsrc-google

src/main/java/com/google/devtools/build/lib/sandbox/SandboxModule.java

@@ -607,14 +607,17 @@ public void afterCommand() {
 
     SandboxOptions options = env.getOptions().getOptions(SandboxOptions.class);
     int asyncTreeDeleteThreads = options != null ? options.asyncTreeDeleteIdleThreads : 0;
-    if (treeDeleter != null && asyncTreeDeleteThreads > 0) {
-      // If asynchronous deletions were requested, they may still be ongoing so let them be: trying
-      // to delete the base tree synchronously could fail as we can race with those other deletions,
-      // and scheduling an asynchronous deletion could race with future builds.
-      AsynchronousTreeDeleter treeDeleter =
-          (AsynchronousTreeDeleter) checkNotNull(this.treeDeleter);
+
+    // If asynchronous deletions were requested, they may still be ongoing so let them be: trying
+    // to delete the base tree synchronously could fail as we can race with those other deletions,
+    // and scheduling an asynchronous deletion could race with future builds.
+    if (asyncTreeDeleteThreads > 0 && treeDeleter instanceof AsynchronousTreeDeleter) {
+      AsynchronousTreeDeleter treeDeleter = (AsynchronousTreeDeleter) this.treeDeleter;
       treeDeleter.setThreads(asyncTreeDeleteThreads);
     }
+    // `treeDeleter` might not be an AsynchronousTreeDeleter if the user changed the option but
+    // then interrupted the build before the start of the execution phase. But that's OK, there
+    // will be nothing new to delete. See #13240.
 
     if (shouldCleanupSandboxBase) {
       try {