The issue was caused by combination of following factors:
- Clipboard cleanup function will pass node tree as NULL to node free
function.
This is fine on it's own, we don't have tree in clipboard.
- Node free function will call node storage cleanup only when there is
a non-NULL node tree.
This is somewhat weird, because storage cleanup does not take node
tree as argument.
So the solution here: move node storage cleanup outside of check that
node tree is not NULL.
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