[GTK] Only import wnck on X11 display

Wnck is only supported on X11 and raises errors in Wayland so only load
it when X11 present.

Fixes: #3265

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Gtk UI
 
+- Fix errors running on Wayland (#3265).
 - Fix Peers Tab tooltip and context menu errors (#3266).
 
 ### Web UI

deluge/ui/gtk3/mainwindow.py

@@ -13,6 +13,7 @@ import logging
 import os.path
 from hashlib import sha1 as sha
 
+import gi
 from gi.repository import Gtk
 from gi.repository.Gdk import DragAction, WindowState
 from twisted.internet import reactor
@@ -27,19 +28,19 @@ from .common import get_deluge_icon
 from .dialogs import PasswordDialog
 from .ipcinterface import process_args
 
-try:
-    import gi
-
-    gi.require_version('Wnck', '3.0')
-    from gi.repository import Wnck
-except ValueError:
-    Wnck = None
-
 try:
     from gi.repository import GdkX11
 except ImportError:
     GdkX11 = None
 
+Wnck = None
+if GdkX11:
+    try:
+        gi.require_version('Wnck', '3.0')
+        from gi.repository import Wnck
+    except (ImportError, ValueError):
+        pass
+
 log = logging.getLogger(__name__)