fix: use addWidget return value for tabbed group on page 2

In multi-page mode, addWidget routes to the page's widget list and
returns early without appending to d.Widgets. Using d.Widgets{end}
after addWidget on page 2 failed with "Array indices must be positive
integers" because d.Widgets was empty.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HanSur94

examples/example_dashboard_advanced.m

@@ -222,9 +222,8 @@
 % All subsequent addWidget calls route to page 2.
 
 %% Feature 7: GroupWidget tabbed mode — multiple views sharing one space
-d.addWidget('group', 'Label', 'Sensor Distributions', 'Mode', 'tabbed', ...
+g = d.addWidget('group', 'Label', 'Sensor Distributions', 'Mode', 'tabbed', ...
     'Position', [1 1 24 6]);
-g = d.Widgets{end};
 g.addChild(HistogramWidget('Sensor', sTemp, 'Title', 'Temperature Distribution', ...
     'ShowNormalFit', true, 'NumBins', 40), 'Temperature');
 g.addChild(HistogramWidget('Sensor', sPress, 'Title', 'Pressure Distribution', ...