fix NPE in model validation code, better error message

com.unity.ml-agents/Runtime/Communicator/RpcCommunicator.cs

     /// Responsible for communication with External using gRPC.
     internal class RpcCommunicator : ICommunicator
     {
-        // The python package version must be >= s_MinSupportedPythonPackageVersion
-        // and <= s_MaxSupportedPythonPackageVersion.
-        static Version s_MinSupportedPythonPackageVersion = new Version("0.16.1");
-        static Version s_MaxSupportedPythonPackageVersion = new Version("0.20.0");
 
         public event QuitCommandHandler QuitCommandReceived;
         public event ResetCommandHandler ResetCommandReceived;
                 return false;
             }
 
-            if (pythonVersion < s_MinSupportedPythonPackageVersion ||
-                pythonVersion > s_MaxSupportedPythonPackageVersion)
+            if (pythonVersion < PythonTrainerVersions.s_MinSupportedVersion ||
+                pythonVersion > PythonTrainerVersions.s_MaxSupportedVersion)
             {
                 return false;
             }
                         "It is strongly recommended that you use a Python package between {1} and {2}. " +
                         "Training will proceed, but the output format may be different.",
                         pythonPackageVersion,
-                        s_MinSupportedPythonPackageVersion,
-                        s_MaxSupportedPythonPackageVersion
+                        PythonTrainerVersions.s_MinSupportedVersion,
+                        PythonTrainerVersions.s_MaxSupportedVersion
                     );
                 }
             }

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

+using System;
+
 namespace Unity.MLAgents
 {
     /// <summary>
         Default = 0,
         Sensors = 50
     }
+
+    internal static class PythonTrainerVersions
+    {
+        // The python package version must be >= s_MinSupportedPythonPackageVersion
+        // and <= s_MaxSupportedPythonPackageVersion.
+        internal static Version s_MinSupportedVersion = new Version("0.16.1");
+        internal static Version s_MaxSupportedVersion = new Version("0.20.0");
+    }
+
 }

com.unity.ml-agents/Runtime/Inference/BarracudaModelParamLoader.cs

             }
 
             var modelApiVersion = (int)model.GetTensorByName(TensorNames.VersionNumber)[0];
-            var memorySize = (int)model.GetTensorByName(TensorNames.MemorySize)[0];
-            var isContinuousInt = (int)model.GetTensorByName(TensorNames.IsContinuousControl)[0];
-            var isContinuous = GetActionType(isContinuousInt);
-            var actionSize = (int)model.GetTensorByName(TensorNames.ActionOutputShape)[0];
             if (modelApiVersion == -1)
             {
                 failedModelChecks.Add(
 
             if (modelApiVersion > k_ApiVersion)
             {
+                var minTrainerVersion = PythonTrainerVersions.s_MinSupportedVersion;
+                var maxTrainerVersion = PythonTrainerVersions.s_MaxSupportedVersion;
+
-                    "Either retrain with an older trainer, or update to a newer version of com.unity.ml-agents.\n" +
+                    $"Either retrain with an older trainer (between versions {minTrainerVersion} and {maxTrainerVersion}), " +
+                    $"or update to a newer version of com.unity.ml-agents.\n" +
+
+            var memorySize = (int)model.GetTensorByName(TensorNames.MemorySize)[0];
+            var isContinuousInt = (int)model.GetTensorByName(TensorNames.IsContinuousControl)[0];
+            var isContinuous = GetActionType(isContinuousInt);
+            var actionSize = (int)model.GetTensorByName(TensorNames.ActionOutputShape)[0];
 
             failedModelChecks.AddRange(
                 CheckIntScalarPresenceHelper(new Dictionary<string, int>()