fix: add null check for GetMethod reflection call

Co-authored-by: alirezanet <7004080+alirezanet@users.noreply.github.com>

Author: Copilot

src/Gridify/GridifyMapper.cs

@@ -276,8 +276,12 @@ private IGridifyMapper<T> AddNestedMapperWithMapperType<TMapper>(
 
       // Call the internal method using reflection
       var method = typeof(GridifyMapper<T>)
-         .GetMethod(nameof(AddNestedMapperInternal), BindingFlags.NonPublic | BindingFlags.Instance)!
-         .MakeGenericMethod(propertyType);
+         .GetMethod(nameof(AddNestedMapperInternal), BindingFlags.NonPublic | BindingFlags.Instance);
+
+      if (method == null)
+         throw new InvalidOperationException($"Internal method {nameof(AddNestedMapperInternal)} not found");
+
+      method = method.MakeGenericMethod(propertyType);
 
       return (IGridifyMapper<T>)method.Invoke(this, new object?[] { typedLambda, nestedMapper, prefix, overrideIfExists })!;
    }