[smuxi/stable] Engine: log null values in UserConfig.set_Item[]

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit 929a2925d4b2c7c4abffc47f5f083d2b6d7b079a
Author: Mirco Bauer <meebey meebey net>
Date:   Tue Jun 8 20:24:59 2021 +0800

    Engine: log null values in UserConfig.set_Item[]
    
    The Set() method will throw an ArgumentNullException for null values. To aid
    debugging for issues in the config system, the config key is now logged.

 src/Engine/Config/UserConfig.cs | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/src/Engine/Config/UserConfig.cs b/src/Engine/Config/UserConfig.cs
index 5f71a07e..d1261bdd 100644
--- a/src/Engine/Config/UserConfig.cs
+++ b/src/Engine/Config/UserConfig.cs
@@ -98,6 +98,11 @@ namespace Smuxi.Engine
                 return obj;
             }
             set {
+#if LOG4NET
+                if (value == null) {
+                    _Logger.Error($"set_Item[{key}]: value is null");
+                }
+#endif
                 Set(key, value);
             }
         }