o.c.trends.databrowser: Fix NPE when removing item

This error was added with the auto-history-fresh implementation.
o.c.trends.databrowser2: Tab/space cleanup
o.c.trends.databrowser: @inheritDoc to make inherited javadoc more
obvious

.. because that one mentions Model may be null
Merge pull request #562 from ControlSystemStudio/bug560

Data Browser NullPointerException when deleting item