unable to generate test cases (panic: invalid memory address (StdSizes nil) when running on go file)

ran this:
```
~/go/bin/nxt_unit \
-file_path=/home/user/project/message.go \
-directory_path=/home/user/project \
-receiver_name=Message \
-function_name=Decode \
-receiver_is_start \
-usage=plugin
```
crashes with:

```
panic: runtime error: invalid memory address or nil pointer dereference
go/types.(*StdSizes).Sizeof(0x0, ...)
```

pretty sure it's because StdSizes is nil

Start with the supplied nxt_unit command against /home/user/project/message.go and trace the Go type-checking path that reaches go/types.(*StdSizes).Sizeof. No project file or test is named in the report; done means the command completes without the nil-pointer panic and generates the requested test cases.

Written by the indexing model from the issue text.