fix: adding null checks when accessing NetworkBehaviour for develop 2.0.0 (#3012)

* adding null checks when accessing NetworkBehaviour

* updating PR number in changelog

Author: fluong6

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue by adding null checks in `NetworkVariableBase.CanClientRead` and `NetworkVariableBase.CanClientWrite` methods to ensure safe access to `NetworkBehaviour`. (#3012)
 - Fixed issue where `FixedStringSerializer<T>` was using `NetworkVariableSerialization<byte>.AreEqual` to determine if two bytes were equal causes an exception to be thrown due to no byte serializer having been defined. (#3009)
 - Fixed Issue where a state with dual triggers, inbound and outbound, could cause a false layer to layer state transition message to be sent to non-authority `NetworkAnimator` instances and cause a warning message to be logged. (#3008)
 - Fixed issue using collections within `NetworkVariable` where the collection would not detect changes to items or nested items. (#3004)

com.unity.netcode.gameobjects/Runtime/NetworkVariable/NetworkVariableBase.cs

@@ -264,6 +264,11 @@ public virtual bool IsDirty()
         /// <returns>Whether or not the client has permission to read</returns>
         public bool CanClientRead(ulong clientId)
         {
+            if (!m_NetworkBehaviour)
+            {
+                return false;
+            }
+
             // When in distributed authority mode, everyone can read (but only the owner can write)
             if (m_NetworkManager != null && m_NetworkManager.DistributedAuthorityMode)
             {
@@ -286,6 +291,11 @@ public bool CanClientRead(ulong clientId)
         /// <returns>Whether or not the client has permission to write</returns>
         public bool CanClientWrite(ulong clientId)
         {
+            if (!m_NetworkBehaviour)
+            {
+                return false;
+            }
+
             switch (WritePerm)
             {
                 default: