Only update theme if value provided (#5240) (#5241)

- Handles case where null or invalid value provided

(cherry picked from commit 41167f22c9)

Co-authored-by: Oliver <oliver.henry.walters@gmail.com>

InvenTree/InvenTree/views.py

@@ -640,8 +640,12 @@ class AppearanceSelectView(RedirectView):
             user_theme = common_models.ColorTheme()
             user_theme.user = request.user
 
+        if theme:
+            try:
                 user_theme.name = theme
                 user_theme.save()
+            except Exception:
+                pass
 
         return redirect(reverse_lazy('settings'))