fixes

certmgr/Core/Converters/Impl/StringConverter.cs

@@ -4,7 +4,7 @@ public sealed class StringConverter : ValueConverter<string>
 {
     protected override Task<string?> DoConvertAsync(string rawValue, Type targetType, CancellationToken cancellationToken)
     {
-        return Task.FromResult(rawValue);
+        return Task.FromResult((string?)rawValue);
     }
 }
 

certmgr/Core/Converters/Impl/TimeSpanConverter.cs

@@ -15,7 +15,11 @@ public sealed class TimeSpanConverter : ValueConverter<TimeSpan?>
 
         TimeSpan? result;
 
-        if (int.TryParse(valueSpan, out int value))
+        if (TimeSpan.TryParse(valueSpan, out TimeSpan tmp))
+        {
+            result = tmp;
+        }
+        else if (int.TryParse(valueSpan, out int value))
         {
             switch (unit)
             {