fix null reference in model validation code, better error message (#5350)

* fix NPE in model validation code, better error message

* changelog

* fix comment

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Bug Fixes
+#### com.unity.ml-agents (C#)
+- Fixed a null reference exception that occurred when loading an ONNX model file that was generated with a new
+version of the Python trainer (0.26.0 or newer).
+
 ## [1.0.7] - 2021-03-04
 ### Minor Changes
 #### com.unity.ml-agents (C#)

com.unity.ml-agents/Runtime/Constants.cs

 
     internal static class PythonTrainerVersions
     {
-        // The python package version must be >= s_MinSupportedPythonPackageVersion
-        // and <= s_MaxSupportedPythonPackageVersion.
+        // The python package version must be >= s_MinSupportedVersion
+        // and <= s_MaxSupportedVersion.
         internal static Version s_MinSupportedVersion = new Version("0.16.1");
         internal static Version s_MaxSupportedVersion = new Version("0.20.0");
     }