qt: SettingsDialog: guard self.network access

Check if self.network before trying to access it. This would trigger an
exception when toggling the trampoline checkbox in offline mode:
```
 29.13 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "/home/user/Documents/electrum/electrum/gui/qt/settings_dialog.py", line 133, in on_trampoline_checked
    self.network.run_from_another_thread(
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'run_from_another_thread'
 31.00 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "/home/user/Documents/electrum/electrum/gui/qt/settings_dialog.py", line 131, in on_trampoline_checked
    self.network.start_gossip()
    ^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'start_gossip'
```

electrum/gui/qt/settings_dialog.py

@@ -127,11 +127,12 @@ class SettingsDialog(QDialog, QtEventListener):
                     trampoline_cb.setCheckState(Qt.CheckState.Checked)
                     return
             self.config.LIGHTNING_USE_GOSSIP = not use_trampoline
-            if not use_trampoline:
-                self.network.start_gossip()
-            else:
-                self.network.run_from_another_thread(
-                    self.network.stop_gossip())
+            if self.network:
+                if not use_trampoline:
+                    self.network.start_gossip()
+                else:
+                    self.network.run_from_another_thread(
+                        self.network.stop_gossip())
             util.trigger_callback('ln_gossip_sync_progress')
             # FIXME: update all wallet windows
             util.trigger_callback('channels_updated', self.wallet)