not all events have Add method

fixes pythonnet#2405

Author: lostmsu

src/runtime/ClassManager.cs

@@ -290,11 +290,13 @@ internal static void InitClassBase(Type type, ClassBase impl, ReflectedClrType p
 
         internal static bool ShouldBindMethod(MethodBase mb)
         {
+            if (mb is null) throw new ArgumentNullException(nameof(mb));
             return (mb.IsPublic || mb.IsFamily || mb.IsFamilyOrAssembly);
         }
 
         internal static bool ShouldBindField(FieldInfo fi)
         {
+            if (fi is null) throw new ArgumentNullException(nameof(fi));
             return (fi.IsPublic || fi.IsFamily || fi.IsFamilyOrAssembly);
         }
 
@@ -326,7 +328,7 @@ internal static bool ShouldBindProperty(PropertyInfo pi)
 
         internal static bool ShouldBindEvent(EventInfo ei)
         {
-            return ShouldBindMethod(ei.GetAddMethod(true));
+            return ei.GetAddMethod(true) is { } add && ShouldBindMethod(add);
         }
 
         private static ClassInfo GetClassInfo(Type type, ClassBase impl)