Protected from null argument

Following neat feedback from Christian.

Issue #384

src/main/java/org/mockito/internal/framework/DefaultMockitoFramework.java

@@ -21,12 +21,20 @@ public void setStubbingListener(StubbingListener listener) {
     }
 
     public void addListener(MockitoListener listener) {
+        assertNotNull(listener);
         if (listener instanceof MockCreationListener) {
             mockingProgress().addListener(listener);
         }
     }
 
     public void removeListener(MockitoListener listener) {
+        assertNotNull(listener);
         mockingProgress().removeListener(listener);
     }
+
+    private void assertNotNull(MockitoListener listener) {
+        if (listener == null) {
+            throw new IllegalArgumentException("listener cannot be null");
+        }
+    }
 }

src/test/java/org/mockito/internal/framework/DefaultMockitoFrameworkTest.java

@@ -0,0 +1,19 @@
+package org.mockito.internal.framework;
+
+import org.junit.Test;
+import org.mockitoutil.TestBase;
+
+public class DefaultMockitoFrameworkTest extends TestBase {
+
+    DefaultMockitoFramework framework = new DefaultMockitoFramework();
+
+    @Test(expected = IllegalArgumentException.class)
+    public void prevents_adding_null_listener() {
+        framework.addListener(null);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void prevents_removing_null_listener() {
+        framework.addListener(null);
+    }
+}