Add a workaround for unbinding script members on Mono

OpenRA.Game/Scripting/ScriptObjectWrapper.cs

@@ -24,6 +24,10 @@ namespace OpenRA.Scripting
 		protected readonly ScriptContext Context;
 		readonly Dictionary<string, ScriptMemberWrapper> members = new Dictionary<string, ScriptMemberWrapper>();
 
+#if !NET5_0_OR_GREATER
+		readonly List<string> membersToRemove = new List<string>();
+#endif
+
 		public ScriptObjectWrapper(ScriptContext context)
 		{
 			Context = context;
@@ -63,9 +67,22 @@ namespace OpenRA.Scripting
 
 		protected void Unbind(Type targetType)
 		{
+#if NET5_0_OR_GREATER
+			// NOTE: In newer versions of .NET modifying the collection by calling Remove while iterating over it is valid
 			foreach (var m in members)
 				if (targetType == m.Value.Target.GetType())
 					members.Remove(m.Key);
+#else
+			// PERF: Re-use instead of allocating a new list on each unbind
+			membersToRemove.Clear();
+
+			foreach (var m in members)
+				if (targetType == m.Value.Target.GetType())
+					membersToRemove.Add(m.Key);
+
+			foreach (var m in membersToRemove)
+				members.Remove(m);
+#endif
 		}
 
 		public bool ContainsKey(string key) { return members.ContainsKey(key); }