payserver: better handle running with --offline

```
 19.70 | E | plugin | Plugin error. plugin: payserver, hook: wallet_export_request
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/plugin.py", line 227, in run_hook
    r = f(*args)
  File "/home/user/wspace/electrum/electrum/plugins/payserver/payserver.py", line 69, in wallet_export_request
    d['view_url'] = self.view_url(key)
  File "/home/user/wspace/electrum/electrum/plugins/payserver/payserver.py", line 55, in view_url
    return self.server.base_url + self.server.root + '/pay?id=' + key
AttributeError: 'NoneType' object has no attribute 'base_url'
```
```
 23.22 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/plugins/payserver/qt.py", line 48, in settings_dialog
    url = self.server.base_url + self.server.root + '/create_invoice.html'
AttributeError: 'NoneType' object has no attribute 'base_url'
```

electrum/plugins/payserver/payserver.py

@@ -26,7 +26,7 @@
 import os
 import asyncio
 from collections import defaultdict
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from aiohttp import web
 from aiorpcx import NetAddress
@@ -50,7 +50,9 @@ class PayServerPlugin(BasePlugin):
         self.config = config
         self.server = None
 
-    def view_url(self, key):
+    def view_url(self, key) -> Optional[str]:
+        if not self.server:
+            return None
         return self.server.base_url + self.server.root + '/pay?id=' + key
 
     @hook
@@ -65,7 +67,8 @@ class PayServerPlugin(BasePlugin):
 
     @hook
     def wallet_export_request(self, d, key):
-        d['view_url'] = self.view_url(key)
+        if view_url := self.view_url(key):
+            d['view_url'] = view_url
 
 
 class PayServer(Logger, EventListener):

electrum/plugins/payserver/qt.py

@@ -42,9 +42,13 @@ class Plugin(PayServerPlugin):
             partial(self.settings_dialog, window))
 
     def settings_dialog(self, window: WindowModalDialog):
+        if self.config.get('offline'):
+            window.show_error(_("You are offline."))
+            return
         d = WindowModalDialog(window, _("PayServer Settings"))
         form = QtWidgets.QFormLayout(None)
         addr = self.config.get('payserver_address', 'localhost:8080')
+        assert self.server
         url = self.server.base_url + self.server.root + '/create_invoice.html'
         self.help_button = QtWidgets.QPushButton('View sample invoice creation form')
         self.help_button.clicked.connect(lambda: webopen(url))