Add missing unsafe block to check for main thread macOS

src/platform/macos/window.rs

@@ -197,9 +197,11 @@ impl Window {
                pl_attribs: &PlatformSpecificWindowBuilderAttributes)
                -> Result<Window, CreationError>
     {
+        unsafe {
             if !msg_send![cocoa::base::class("NSThread"), isMainThread] {
                 panic!("Windows can only be created on the main thread on macOS");
             }
+        }
 
         let app = match Window::create_app(pl_attribs.activation_policy) {
             Some(app) => app,