fix bug introduced in last commit

pathpy/classes/multi_order_model.py

@@ -70,7 +70,7 @@ def max_order(self):
         """the current maximum order of the multi-order model"""
         orders = list(self.layers.keys())
         if not orders:
-            return -1
+            return None
         else:
             return max(orders)
 
@@ -129,13 +129,14 @@ def add_layers(self, max_order):
         """
         from pathpy import ENABLE_MULTICORE_SUPPORT
 
+        current_max_order = self.max_order if self.max_order else -1
         if max_order < 0:
             raise PathpyError("max_order must be a positive integer not %d" % max_order)
 
-        if max_order <= self.max_order:
+        if max_order <= current_max_order:
             Log.add("Layers up to order %d already added. Nothing changed." % self.max_order)
 
-        orders_to_add = list(range(self.max_order+1, max_order+1))
+        orders_to_add = list(range(current_max_order+1, max_order+1))
         if len(orders_to_add) > 1 and ENABLE_MULTICORE_SUPPORT:
             self.__add_layers_parallel(orders_to_add)
         else: