show error message when parsing empty tx

gui/qt/main_window.py

@@ -2018,9 +2018,8 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         try:
             tx = tx_from_str(txt)
             return Transaction(tx)
-        except:
+        except BaseException as e:
-            traceback.print_exc(file=sys.stdout)
+            self.show_critical(_("Electrum was unable to parse your transaction") + ":\n" + str(e))
-            self.show_critical(_("Electrum was unable to parse your transaction"))
             return
 
     def read_tx_from_qrcode(self):

lib/transaction.py

@@ -896,6 +896,8 @@ def tx_from_str(txt):
     "json or raw hexadecimal"
     import json
     txt = txt.strip()
+    if not txt:
+        raise ValueError("empty string")
     try:
         bfh(txt)
         is_hex = True