Prevent empty explorations from crashing

[ahuang11]

For a new user exploring the UI, randomly clicking on the tabs would crash the UI if they haven't input any message yet.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.85%. Comparing base (5d347d5) to head (dff8e38).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/ui.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #912      +/-   ##
==========================================
- Coverage   56.86%   56.85%   -0.01%     
==========================================
  Files         108      108              
  Lines       13720    13722       +2     
==========================================
  Hits         7802     7802              
- Misses       5918     5920       +2     

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