Looks like issued by wx, not cb. If it is actually an assert, no wonder why it pops up in DEBUG only.
For further readers: It is the horizontal sizer which contains the label "select application appearance" and the combo-box where you select between default, dark and lite mode.
It makes sense to me when reading the corresponding wx doc:
https://docs.wxwidgets.org/3.2/overview_sizer.html#overview_sizer_boxCorrespondingly, when using a horizontal sizer, each child can be centered, aligned at the bottom or aligned at the top.
Being no expert on wx whatsoever, I estimate, that just the flag wxALIGN_CENTER_HORIZONTAL should be deleted, not the whole line.
Pulling the window larger the static label remains the same size while the combo box grows. That's fine for me. So, probably no need for a spacer.
The corresponding snippet around trunk/src/src/resources/env_settings.xrc:768 is:
<object class="sizeritem">
<object class="wxBoxSizer">
<object class="sizeritem">
<object class="wxStaticText" name="wxID_ANY">
<label>Select application appearance (if supported by platform):</label>
</object>
<flag>wxLEFT|wxRIGHT|wxALIGN_CENTER_HORIZONTAL|wxALIGN_CENTER_VERTICAL</flag>
<border>4</border>
</object>
<object class="sizeritem">
<object class="wxChoice" name="chAppearance">
<content>
<item>System default</item>
<item>Dark</item>
<item>Light</item>
</content>
<selection>0</selection>
<enabled>0</enabled>
</object>
<flag>wxALIGN_CENTER_VERTICAL</flag>
<border>4</border>
<option>1</option>
</object>
</object>
<flag>wxEXPAND</flag>
<border>4</border>
</object>