app/internal/wm: [macOS] avoid crash in window.Wakeup

window.Wakeup assumes the window.w field is never reset to nil. Avoid
doing that in gio_onClose.

While here, ensure a valid window handle in window.Close by calling
the checked window.runOnMain method, not the bare runOnMain function.

Fixes gio#258

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/wm/os_macos.go

@@ -287,7 +287,7 @@ func (w *window) Close() {
 	// causing a deadlock because Close is called during an event.
 	// Break the deadlock by deferring the close, making Close more
 	// akin to a message like the other platforms.
-	go runOnMain(func() {
+	go w.runOnMain(func() {
 		C.closeWindow(w.window)
 	})
 }
@@ -425,7 +425,9 @@ func gio_onClose(view C.CFTypeRef) {
 	w.displayLink.Close()
 	C.CFRelease(w.view)
 	C.CFRelease(w.window)
-	*w = window{}
+	w.view = 0
+	w.window = 0
+	w.displayLink = nil
 }
 
 //export gio_onHide