[FIX] owheatmap: Fix assertion error when restoring selection

[ales-erjavec]

Issue

Heatmap widget raises an assertion error when restoring selection and the input data has only a single column and columns clustering is selected.

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/Documents/workspace/orange-canvas/orangecanvas/scheme/signalmanager.py", line 1125, in __process_next
    if self.__process_next_helper(use_max_active=True):
  File "/Users/aleserjavec/Documents/workspace/orange-canvas/orangecanvas/scheme/signalmanager.py", line 1164, in __process_next_helper
    self.process_node(selected_node)
  File "/Users/aleserjavec/Documents/workspace/orange-canvas/orangecanvas/scheme/signalmanager.py", line 707, in process_node
    self.send_to_node(node, signals_in)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/workflow/widgetsscheme.py", line 805, in send_to_node
    self.process_signals_for_widget(node, widget, signals)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/workflow/widgetsscheme.py", line 819, in process_signals_for_widget
    process_signals_for_widget(widget, signals, workflow)
  File "/Users/aleserjavec/.local/opt/python3.8/Python.framework/Versions/3.8/lib/python3.8/functools.py", line 875, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/workflow/widgetsscheme.py", line 920, in process_signals_for_widget
    process_signal_input(input_meta, widget, signal, workflow)
  File "/Users/aleserjavec/.local/opt/python3.8/Python.framework/Versions/3.8/lib/python3.8/functools.py", line 875, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/workflow/widgetsscheme.py", line 883, in process_signal_input_default
    notify_input_helper(
  File "/Users/aleserjavec/.local/opt/python3.8/Python.framework/Versions/3.8/lib/python3.8/functools.py", line 875, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/utils/signals.py", line 643, in set_input_helper
    handler(*args)
  File "/Users/aleserjavec/Documents/workspace/orange-widget-base/orangewidget/utils/signals.py", line 198, in summarize_wrapper
    method(widget, value)
  File "/Users/aleserjavec/Documents/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 665, in set_dataset
    assert self.scene.widget is not None
AssertionError
-------------------------------------------------------------------------------

Easiest way to test is to File -> Heatmap, select column clustering and select a couple of rows in the heatmap, then duplicate/copy/paste the Heatmap, add File -> Select Columns, select single column in Select Columns and connect Select Columns -> Heatmap (2)

Description of changes

Fix assertion error when restoring selection

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #5517 (95b35c5) into master (f9d78d6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5517      +/-   ##
==========================================
- Coverage   86.38%   86.37%   -0.01%     
==========================================
  Files         304      304              
  Lines       61753    61753              
==========================================
- Hits        53344    53341       -3     
- Misses       8409     8412       +3