UI: Keymap: Show alert when operator or property not found

When a keymap operator is not found, color the label and input field
in red. This can help identify issues when migrating preferences and
some operators no longer exist.

Screenshots in the pull request.

Pull Request: https://projects.blender.org/blender/blender/pulls/130675

scripts/modules/rna_keymap_ui.py

@@ -19,6 +19,17 @@ from bpy.app.translations import (
 )
 
 
+def _is_operator_available(idname):
+    module, _, operator = idname.partition(".")
+
+    # Check if the module and operator exist.
+    return (
+        module and
+        operator and
+        getattr(getattr(bpy.ops, module, None), operator, None) is not None
+    )
+
+
 def _indented_layout(layout, level):
     indentpx = 16
     if level == 0:
@@ -110,6 +121,7 @@ def draw_km(display_keymaps, kc, km, children, layout, level):
 
 def draw_kmi(display_keymaps, kc, km, kmi, layout, level):
     map_type = kmi.map_type
+    is_op_available = _is_operator_available(kmi.idname)
 
     col = _indented_layout(layout, level)
 
@@ -128,9 +140,19 @@ def draw_kmi(display_keymaps, kc, km, kmi, layout, level):
 
     if km.is_modal:
         row.separator()
+        row.alert = not kmi.propvalue
         row.prop(kmi, "propvalue", text="")
     else:
-        row.label(text=kmi.name)
+        if is_op_available:
+            row.label(text=kmi.name)
+        # The default item when adding a new item is "none"
+        # so consider this unassigned along with an empty string.
+        elif kmi.idname in {"none", ""}:
+            row.alert = True
+            row.label(text="(Unassigned)")
+        else:
+            row.alert = True
+            row.label(text="{:s} (unavailable)".format(kmi.idname), icon='WARNING_LARGE')
 
     row = split.row()
     row.prop(kmi, "map_type", text="")
@@ -173,9 +195,12 @@ def draw_kmi(display_keymaps, kc, km, kmi, layout, level):
         sub = split.row()
 
         if km.is_modal:
+            sub.alert = not kmi.propvalue
             sub.prop(kmi, "propvalue", text="")
         else:
-            sub.prop(kmi, "idname", text="")
+            subrow = sub.row()
+            subrow.alert = not is_op_available
+            subrow.prop(kmi, "idname", text="", placeholder="Operator")
 
         if map_type not in {'TEXTINPUT', 'TIMER'}:
             from sys import platform