fix #272 fix error on concentrator list

[Sben65]

Description

What's new?

• Bug: Error when accessing to Concentrator list #272

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Tests
• Other

[codecov]

Codecov Report

Merging #295 (74a7c1e) into main (5c23278) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #295      +/-   ##
==========================================
+ Coverage   39.62%   39.70%   +0.08%     
==========================================
  Files          93       93              
  Lines        2751     2755       +4     
==========================================
+ Hits         1090     1094       +4     
  Misses       1661     1661              

Impacted Files	Coverage Δ
...ers/v1.0/LoRaWAN/LoRaWANConcentratorsController.cs	75.00% <100.00%> (+1.66%)	⬆️

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