SYN-5749: AttributeError type error with getting deprecated property

[ghost]

• Added type checks for the appropriate modelext functions
• Added new test to validate type checking

Resolves SYN-5749

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.11 ⚠️

Comparison is base (024d9f0) 97.30% compared to head (b83c061) 97.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3243      +/-   ##
==========================================
- Coverage   97.30%   97.20%   -0.11%     
==========================================
  Files         226      226              
  Lines       45174    45198      +24     
==========================================
- Hits        43958    43936      -22     
- Misses       1216     1262      +46     

Flag	Coverage Δ
linux	97.20% <100.00%> (+<0.01%)	⬆️
linux_replay	?

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Impacted Files	Coverage Δ
synapse/cortex.py	96.92% <100.00%> (-0.49%)	⬇️

... and 8 files with indirect coverage changes

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