Handle invalid PIN on exporting private key

Prevent Android App from crashing when a wrong PIN is entered.

gui/kivy/main_window.py

@@ -928,6 +928,10 @@ class ElectrumWindow(App):
                 return
             if not self.wallet.can_export():
                 return
-            key = str(self.wallet.export_private_key(addr, password)[0])
-            pk_label.data = key
+            try:
+                key = str(self.wallet.export_private_key(addr, password)[0])
+                pk_label.data = key
+            except InvalidPassword:
+                self.show_error("Invalid PIN")
+                return
         self.protected(_("Enter your PIN code in order to decrypt your private key"), show_private_key, (addr, pk_label))