[GR-39857] Set nativeInterruptTimer to null at the end of pre-initialization

PullRequest: truffleruby/3437

Author: eregon

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -121,6 +121,7 @@ public void initialize() {
     public void dispose() {
         if (nativeInterrupt) {
             nativeInterruptTimer.cancel();
+            nativeInterruptTimer = null;
         }
     }