qml: don't initialize swaphelper if a non-lightning wallet is opened

Due to swaphelper's lifecycle (it is kept around longer than the swap dialog) it might
get initialized with a non-lightning wallet. don't initialize in that case.

proper fix is to tie the lifecycle to the swap process, or make it a child of the wallet.

electrum/gui/qml/qeswaphelper.py

@@ -182,6 +182,8 @@ class QESwapHelper(AuthMixin, QObject):
 
     def init_swap_slider_range(self):
         lnworker = self._wallet.wallet.lnworker
+        if not lnworker:
+            return
         swap_manager = lnworker.swap_manager
         try:
             asyncio.run(swap_manager.get_pairs())