Fix Bug 873

[Davknapp]

Cause of the error:
During partitioning of the cmesh vertex-data of the tree can be moved around. If the active tree of the geometry is the same as befor partitioning the vertex-data has been moved without notifying the geometry.
Solution:
The active tree of the geometry-handler is set to a negative value that is out of bounds of the possible ltreeids. Therefore the active tree is set again as soon as we want to load tree data and the tree-data is actually loaded.

Thanks to @lukasdreyer for helping with the bug-hunt.

Closes #873

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