macOS: don’t crash when wrapping foreign views

window.contentView can be of any NSView subclass.
Get to the QCocoaWindow via QCocoaNSWindow instead.

Change-Id: I8c761fd22e6078b075d8dd035ad767b9e4cb6da2
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qnswindowdelegate.mm

@@ -51,7 +51,9 @@ static QRegExp whitespaceRegex = QRegExp(QStringLiteral("\\s*"));
 
 static QCocoaWindow *toPlatformWindow(NSWindow *window)
 {
-    return qnsview_cast(window.contentView).platformWindow;
+    if ([window conformsToProtocol:@protocol(QNSWindowProtocol)])
+        return static_cast<QCocoaNSWindow *>(window).platformWindow;
+    return nullptr;
 }
 
 @implementation QNSWindowDelegate