[fix] fix raising NullReferenceException in generating MethodBridge for MonoPInvokeCallbackAttribute while constructor arguments is empty.

Author: pirunxi

Editor/MethodBridge/Generator.cs

@@ -559,6 +559,11 @@ private static CallingConvention GetCallingConvention(MethodDef method)
             {
                 return CallingConvention.Winapi;
             }
+            if (monoPInvokeCallbackAttr.ConstructorArguments.Count == 0)
+            {
+                Debug.LogError($"MonoPInvokeCallbackAttribute on method {method.FullName} has no constructor arguments. Using CallingConvention.Winapi as default.");
+                return CallingConvention.Winapi;
+            }
             object delegateTypeSig = monoPInvokeCallbackAttr.ConstructorArguments[0].Value;
 
             TypeDef delegateTypeDef;
@@ -580,7 +585,7 @@ private static CallingConvention GetCallingConvention(MethodDef method)
                 throw new NotSupportedException($"Unsupported delegate type: {delegateTypeSig}");
             }
             var attr = delegateTypeDef.CustomAttributes.FirstOrDefault(ca => ca.AttributeType.FullName == "System.Runtime.InteropServices.UnmanagedFunctionPointerAttribute");
-            if (attr == null)
+            if (attr == null || attr.ConstructorArguments.Count == 0)
             {
                 return CallingConvention.Winapi;
             }