Panic if the user attempts to create window on non-main thread

src/platform/macos/window.rs

@@ -197,6 +197,10 @@ impl Window {
                pl_attribs: &PlatformSpecificWindowBuilderAttributes)
                -> Result<Window, CreationError>
     {
+        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,
             None      => { return Err(OsError(format!("Couldn't create NSApplication"))); },