ConcurrentModificationException on plugin start (reload)

[mPokornyETM]

fix #623

This changes will fixed ConcurrentModificationException on plugin start (reload)

Testing done

as discussed in #623 the test are done manually.

Proposed upgrade guidelines

N/A

Localizations

NN

Submitter checklist

• The Jira / Github issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples).
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  • Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during the upgrade.
• There is automated testing or an explanation that explains why this change has no tests.
• [ ] New public functions for internal use only are annotated with @NoExternalUse. In case it is used by non java code the Used by {@code <panel>.jelly} Javadocs are annotated.
• [ ] New or substantially changed JavaScript is not defined inline and does not call eval to ease the future introduction of Content Security Policy (CSP) directives (see documentation).
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