catch errors

src/urh/models/PLabelTableModel.py

@@ -48,7 +48,10 @@ class PLabelTableModel(QAbstractTableModel):
     def data(self, index: QModelIndex, role=Qt.DisplayRole):
         i, j = index.row(), index.column()
         if role == Qt.DisplayRole:
-            lbl = self.message_type[i]
+            try:
+                lbl = self.message_type[i]
+            except IndexError:
+                return False
             if j == 0:
                 return lbl.name
             elif j == 1:

src/urh/signalprocessing/Signal.py

@@ -238,8 +238,10 @@ class Signal(QObject):
 
     @property
     def real_plot_data(self):
-        return self.data.real
-
+        try:
+            return self.data.real
+        except AttributeError:
+            return np.zeros(0, dtype=np.float32)
     @property
     def wave_data(self):
         return bytearray(np.multiply(-1, (np.round(self.data.real * 127)).astype(np.int8)))