Frictions: Fix none assiciated stricklers at save/load.

src/Model/Friction/Friction.py

@@ -77,7 +77,14 @@ class Friction(SQLSubModel):
         for row in table:
             ind = row[0]
             # Get stricklers
+            if int(row[3]) == -1:
+                bs = None
+            else:
                 bs = next(filter(lambda s: s.id == row[3], stricklers))
+
+            if int(row[4]) == -1:
+                es = None
+            else:
                 es = next(filter(lambda s: s.id == row[4], stricklers))
 
             # Create friction
@@ -94,6 +101,15 @@ class Friction(SQLSubModel):
 
     def _db_save(self, execute, data=None):
         ind = data["ind"]
+
+        b_s_id = -1
+        e_s_id = -1
+
+        if self._begin_strickler is not None:
+            b_s_id = self._begin_strickler.id
+        if self._end_strickler is not None:
+            e_s_id = self._end_strickler.id
+
         execute(
             "INSERT INTO " +
             "friction(ind, begin_kp, end_kp, " +
@@ -101,7 +117,7 @@ class Friction(SQLSubModel):
             "VALUES (" +
             f"{ind}, {self._begin_kp}, {self._end_kp}, " +
             f"{self._edge.id}, " +
-            f"{self._begin_strickler.id}, {self._end_strickler.id}" +
+            f"{b_s_id}, {e_s_id}" +
             ")"
         )