owrank: Add migrate_settings

Fix an error when restoring from a workflow that was saved before
widget supported ranking features on data with no class.

Orange/widgets/data/owrank.py

@@ -77,6 +77,7 @@ class OWRank(OWWidget):
     # Header state for discrete/continuous/no_class scores
     headerState = Setting([None, None, None])
 
+    settings_version = 1
     settingsHandler = DomainContextHandler()
     selected_rows = ContextSetting([])
 
@@ -688,6 +689,19 @@ def create_scores_table(self, labels):
         new_table.name = "Feature Scores"
         return new_table
 
+    @classmethod
+    def migrate_settings(cls, settings, version):
+        super().migrate_settings(settings, version)
+        if not version:
+            # Before fc5caa1e1d716607f1f5c4e0b0be265c23280fa0
+            # headerState had length 2
+            headerState = settings.get("headerState", None)
+            if headerState is not None and \
+                    isinstance(headerState, tuple) and \
+                    len(headerState) < 3:
+                headerState = (list(headerState) + [None] * 3)[:3]
+                settings["headerState"] = headerState
+
 
 class ScoreValueItem(QStandardItem):
     """A StandardItem subclass for python objects.