Only update theme if value provided (#5240)

- Handles case where null or invalid value provided

InvenTree/InvenTree/views.py

@@ -639,8 +639,12 @@ class AppearanceSelectView(RedirectView):
             user_theme = common_models.ColorTheme()
             user_theme.user = request.user
 
-        user_theme.name = theme
-        user_theme.save()
+        if theme:
+            try:
+                user_theme.name = theme
+                user_theme.save()
+            except Exception:
+                pass
 
         return redirect(reverse_lazy('settings'))