[wpilib] Throw early when EventLoop is modified while running (#6115)

wpilibj/src/test/java/edu/wpi/first/wpilibj/event/EventLoopTest.java

@@ -5,7 +5,9 @@
 package edu.wpi.first.wpilibj.event;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.util.ConcurrentModificationException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import org.junit.jupiter.api.Test;
@@ -58,4 +60,33 @@ class EventLoopTest {
     // shouldn't change
     assertEquals(1, counter.get());
   }
+
+  @Test
+  void testConcurrentModification() {
+    var loop = new EventLoop();
+
+    loop.bind(
+        () -> {
+          assertThrows(
+              ConcurrentModificationException.class,
+              () -> {
+                loop.bind(() -> {});
+              });
+        });
+
+    loop.poll();
+
+    loop.clear();
+
+    loop.bind(
+        () -> {
+          assertThrows(
+              ConcurrentModificationException.class,
+              () -> {
+                loop.clear();
+              });
+        });
+
+    loop.poll();
+  }
 }