Fix errors when creating new RayCast sensor (#5261)

* check if tag and angle is null

* add test

* changelog

com.unity.ml-agents/CHANGELOG.md

 settings. Unfortunately, this may require retraining models if it changes the resulting order of the sensors
 or actuators on your system. (#5194)
 - Removed additional memory allocations that were occurring due to assert messages and iterating of DemonstrationRecorders. (#5246)
+- Fixed a bug where agent trying to access unintialized fields when creating a new RayPerceptionSensorComponent on an agent. (#5261)
 
 ## [1.9.1-preview] - 2021-04-13
 ### Major Changes

com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs

         /// <returns></returns>
         public int OutputSize()
         {
-            return (DetectableTags.Count + 2) * Angles.Count;
+            return ((DetectableTags?.Count ?? 0) + 2) * (Angles?.Count ?? 0);
         }
 
         /// <summary>

com.unity.ml-agents/Tests/Runtime/Sensor/RayPerceptionSensorTests.cs

                 Assert.AreEqual(-1, castOutput.RayOutputs[0].HitTagIndex);
             }
         }
+
+        [Test]
+        public void TestCreateDefault()
+        {
+            SetupScene();
+            var obj = new GameObject("agent");
+            var perception = obj.AddComponent<RayPerceptionSensorComponent3D>();
+
+            Assert.DoesNotThrow(() =>
+            {
+                perception.CreateSensors();
+            });
+        }
 #endif
     }
 }