bugfix: Catch route not found in spanner

[codekeyz]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.51%. Comparing base (c03ea26) to head (52bb5e3).

Files with missing lines	Patch %	Lines
packages/spanner/lib/src/tree/tree.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   71.55%   71.51%   -0.05%     
==========================================
  Files          45       45              
  Lines        1533     1534       +1     
==========================================
  Hits         1097     1097              
- Misses        436      437       +1     

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