C# GetOption: throws NullReferenceException

[zhxymh]

What version of protobuf and what language are you using?
Version: v3.11.4
Language: C#

What operating system (Linux, Windows, ...) and version?
mac 10.14.6

What runtime / compiler are you using (e.g., python version or gcc version)
.NET Core 3.1.0

What did you do?
Use method FileDescriptor.GetOption.

If the proto file sets the options, it works. But if it is not set, throws NullReferenceException

System.NullReferenceException : Object reference not set to an instance of an object.
   at Google.Protobuf.Reflection.FileDescriptor.GetOption[T](Extension`2 extension)
   at AElf.FileDescriptorExtensions.GetIdentity(FileDescriptor descriptor) in /Users/zhangxin/code/AElf/src/AElf.Types/Protobuf/FileDescriptorExtensions.cs:line 9

I think we should check if this.Proto.Options is null. If null, the default value is returned.

Google.Protobuf.Reflection.FileDescriptor.cs

[xtroncode]

#7130 Solves this but it hasn't been merged yet.

[zhxymh]

#7130 Solves this but it hasn't been merged yet.

Thx!