Fix regression in DBC files with no attribute "BusType"

Fix regression in DBC files with no attribute "BusType"

Author: SabrineBH

src/canmatrix/formats/fibex.py

@@ -467,8 +467,7 @@ def dump(db, f, **options):
     create_sub_element_fx(cluster, "SPEED", "500")
     create_sub_element_fx(cluster, "IS-HIGH-LOW-BIT-ORDER", "true")
     create_sub_element_fx(cluster, "BIT-COUNTING-POLICY", "MONOTONE")
-    attribute = db.attributes["BusType"]
-    if attribute is not None and attribute == "CAN FD":
+    if "BusType" in db.attributes and db.attributes["BusType"] == "CAN FD":
         protocol = create_sub_element_fx(cluster, "PROTOCOL", "CAN-FD")
         create_sub_element_fx(cluster, "CAN-FD-SPEED", "2000000")
     else: