When custom skin was loaded, the listeners added in `ContextMenuContent` class while loading the default skin were not removed. This was causing the NPE when outdated listeners were invoked.
Updated the code to dispose listeners in the `dispose` method of `ContextMenuSkin` so that when new skin is loaded, listeners added in the old skin are removed. Added system test to validate the fix. ------------- Commit messages: - Add copyright text to fxml - Fix NPE in submenu of context menu Changes: https://git.openjdk.org/jfx/pull/1082/files Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1082&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8088594 Stats: 342 lines in 6 files changed: 324 ins; 17 del; 1 mod Patch: https://git.openjdk.org/jfx/pull/1082.diff Fetch: git fetch https://git.openjdk.org/jfx.git pull/1082/head:pull/1082 PR: https://git.openjdk.org/jfx/pull/1082
