[FIX] OWWidget: Fix an error on mouse press when widget has no basic layout

[ales-erjavec]

Issue

If a widget has no basic layout (want_basic_layout = False) and error is raised on a mouse press
in an empty widget area.

-------------------------- AttributeError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/widget.py", line 830, in mousePressEvent
    self.message_bar.flashIcon()
AttributeError: 'NoneType' object has no attribute 'flashIcon'
-------------------------------------------------------------------------------

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3821 into master will increase coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #3821      +/-   ##
==========================================
+ Coverage   85.06%   85.07%   +<.01%     
==========================================
  Files         374      374              
  Lines       69148    69150       +2     
==========================================
+ Hits        58820    58827       +7     
+ Misses      10328    10323       -5