fix: handle ! amount for payment identifiers

fixes exceptions ocurring when entering a '!' amount with a payment
identifier that is not allowed to send max (e.g. bolt11).
fixes exception when "Max" is still set from previous PI and the PI gets
replaced by one that doesn't allow max sending.

electrum/gui/qt/send_tab.py

@@ -242,9 +242,11 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
 
         if pi is None or pi.type == PaymentIdentifierType.UNKNOWN:
             return
-
-        assert pi.type in [PaymentIdentifierType.SPK, PaymentIdentifierType.MULTILINE,
-                           PaymentIdentifierType.BIP21, PaymentIdentifierType.OPENALIAS]
+        elif pi.type not in [PaymentIdentifierType.SPK, PaymentIdentifierType.MULTILINE,
+                           PaymentIdentifierType.BIP21, PaymentIdentifierType.OPENALIAS]:
+            # clear the amount field once it is clear this PI is not eligible for '!'
+            self.amount_e.clear()
+            return
 
         if pi.type == PaymentIdentifierType.BIP21:
             assert 'amount' not in pi.bip21
@@ -406,6 +408,8 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
             self.amount_e.setFrozen(lock_amount)
         if lock_max is not None:
             self.max_button.setEnabled(not lock_max)
+            if lock_max is True:
+                self.max_button.setChecked(False)
         if lock_description is not None:
             self.message_e.setFrozen(lock_description)
 
@@ -466,6 +470,8 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
         # resolve '!' in amount editor if it was set before PI
         if not lock_max and self.amount_e.text() == '!':
             self.spend_max()
+        elif lock_max and self.amount_e.text() == '!':
+            self.amount_e.clear()
 
         pi_unusable = pi.is_error() or (not self.wallet.has_lightning() and not pi.is_onchain())
         is_spk_script = pi.type == PaymentIdentifierType.SPK and not pi.spk_is_address