check if selectables is None

src/model/entity/Variable.py

@@ -127,6 +127,7 @@ class Variable:
         value = self.eval()
 
         if self.type == VariableType.SELECT_SINGLE:
+            if isinstance(self._selectables, list):
                 for selectable in self.selectables:
                     if selectable.key == value:
                         return str(selectable.label)