Known issue 
Remove Landscape
solution is in there. In short:
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Select landscape in outliner.
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Select all or a subset of the landscape proxies and delete them. Save all.
*repeat these steps until all landscape proxies are deleted -
Select landscape in outliner
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Select all regions and delete them. Save all.
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Select landscape in outliner, delete it. Save all.
It may be needed to close the editor after save all, to remove references that are not cleared.