fix: Add self._messages to BUILDING_GALLERY section in app initialization

[klmcadams]

Add self._messages = None to the BUILDING_GALLERY section of the App initialization so the following AttributeError does not occur when running make -C doc html:

app.messages.show()

if self.messages is not None:
^^^
AttributeError: 'App' object has no attribute '_messages'. Did you mean: 'messages'?

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.12% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (0e72d05)	2537	2191	86.36%
Head commit (b6866da)	2537 (+0)	2194 (+3)	86.48% (+0.12%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1171)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.48%. Comparing base (0e72d05) to head (b6866da).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1171      +/-   ##
==========================================
+ Coverage   86.36%   86.48%   +0.11%     
==========================================
  Files          39       39              
  Lines        2537     2537              
==========================================
+ Hits         2191     2194       +3     
+ Misses        346      343       -3     

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