Increase priority of callback for attribute_display_unit and ensure we don't try and convert None values

Author: astrofrog

glue/core/state_objects.py

@@ -332,7 +332,11 @@ def update_values(self, force=False, use_default_modifiers=False, **properties):
 
         self._previous_units = display_units
 
-        if set(properties) == {'display_units'}:
+        if (
+                set(properties) == {'display_units'} and
+                self.lower is not None and
+                self.upper is not None
+            ):
 
             converter = UnitConverter()
 

glue/viewers/image/state.py

@@ -536,8 +536,7 @@ def format_unit(unit):
 
         ImageLayerState.attribute_display_unit.set_display_func(self, format_unit)
 
-        self.add_callback('attribute', self._update_attribute_display_unit_choices)
-        # self.add_callback('attribute_display_unit', self._convert_attribute_limits_units, echo_old=True)
+        self.add_callback('attribute', self._update_attribute_display_unit_choices, priority=10000)
 
         self._update_attribute_display_unit_choices()