This is an expected result for this particular implementation as it violates Maps's contract: the entrySet() should not return null collection.
Debugger's default renderer for Map objects calls this method to get all entries in the map. You should either use default renderer for class types (right click on the node and select "View As"->"Object") to render this particular instance, or turn "Alternative views for Collections classes" option off, so that generic renderer will be used for collection classes.
Alternatively you may specify your own renderer specifically for Maps that will override the IDEA's one.