fix: avoid access to arbitrary resource via parent

[denis-anisimov]

No description provided.

[vaadin-bot]

SonarQube analysis reported 7 issues

Note: The following issues were found on lines that were not modified in the pull request. Because these issues can't be reported as line comments, they are summarized here:

1. HandlerHelper.java#L204: Define and throw a dedicated exception instead of using a generic one.
2. VaadinBundleTracker.java#L156: Replace the synchronized class "Hashtable" by an unsynchronized one such as "HashMap".
3. VaadinBundleTracker.java#L177: Replace the synchronized class "Hashtable" by an unsynchronized one such as "HashMap".
4. VaadinBundleTracker.java#L281: Reduce the total number of break and continue statements in this loop to use at most one.
5. VaadinBundleTracker.java#L103: Add a "try/catch" block for "openStream".
6. VaadinBundleTracker.java#L104: Add a "try/catch" block for "copy".
7. VaadinBundleTracker.java#L104: Add a "try/catch" block for "getOutputStream".