Fix GetUnderlyingType() can't get Type when Type is NestedCollection

[Hwangwoosam]

in Issue #588 when type is Nested Collection, GetUnderlying Method return null.
The picture below reproduces the problem by temporarily implementing class and executing sample code.

Problem

Below is the result of solving the problem by modifying the GetUnderlyingType() and GetSubType() methods.

Result

[Trimatix]

Hi team, this issue is affecting our applications. What's the status on this PR please?

[Dorrro]

Struggling with the same issue. We would love to see that resolved