tree_view crash when deleting all nodes

[igagis]

. launch app test application
. delete all nodes from tree_view by clicking minus button on them

result: assertion failed:

[!!!fatal] Assertion failed at:
        /home/ivan/prj/morda/src/morda/morda/widgets/group/tree_view.cpp:303| no additional info
tests: /home/ivan/prj/morda/src/morda/morda/widgets/group/tree_view.cpp:303: void morda::tree_view::provider::notify_item_removed(const std::vector<long unsigned int>&): Assertion `false' failed.