macos webview_cocoa.c unknown error

### description
I'm using the master branch in a rust project and it fails to render a webview on macos and the program crashes when instantiating webview
according to my debugger my program just quits when this is hit

Issues lie here:
https://github.com/Boscop/web-view/blob/master/webview-sys/webview_cocoa.c

https://github.com/Boscop/web-view/blob/82d7cbce6228b1a964673cc0f22944ad808eab42/src/lib.rs#L348

## debugger leads

### line 61
```c
if (webview_init(wv) != 0) {
webview_free(wv);
return NULL;
}
return wv;
}
```
### line 328 in webview_cocoa.c
```c
id wkPref = ((id(*)(id, SEL))objc_msgSend)((id)__WKPreferences, sel_registerName("new"));
```

### finally src/lib.rs line 348

```rust
let user_data = Box::new(UserData {
inner: user_data,
live: Arc::new(RwLock::new(())),
invoke_handler: Box::new(invoke_handler),
result: Ok(()),
});
let user_data_ptr = Box::into_raw(user_data);
```



### Somethings up with user_data according to the debugging

Start with webview-sys/webview_cocoa.c at line 328 and src/lib.rs at line 348, then reproduce the macOS webview-instantiation failure from the master branch under a debugger. Trace the webview_init failure path around line 61 to identify why the program quits. Done means the webview renders without crashing during instantiation.

Written by the indexing model from the issue text.