AssertionError on editing links edge case

[irgolic]

Add two file widgets and a test and score,
connect both (Data -> Data, Data -> Test Data),
move the test and score widget such that the two links are close together,
double-click them such that both links' hitboxes are hit (two edit links windows will open),
create a link in one widget that will disconnect the link in the other widget, (e.g., in the Data -> Data widget, create a Data -> Test Data connection)
then delete that one in the other window (e.g., delete the Data -> Test Data connection in the other link).

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File ".../orangecanvas/document/schemeedit.py", line 1634, in __onLinkActivate
    action.edit_links()
  File ".../orangecanvas/document/interactions.py", line 1130, in edit_links
    assert len(links) == 1
AssertionError
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