better error when null cant be converted

Author: SimonCropp

src/Directory.Build.props

@@ -2,7 +2,7 @@
 <Project>
   <PropertyGroup>
     <NoWarn>CS1591;NU5104;CS1573;CS9107;NU1608;NU1109</NoWarn>
-    <Version>31.0.0</Version>
+    <Version>31.0.1</Version>
     <LangVersion>preview</LangVersion>
     <AssemblyVersion>1.0.0</AssemblyVersion>
     <PackageTags>EntityFrameworkCore, EntityFramework, GraphQL</PackageTags>

src/GraphQL.EntityFramework/Where/TypeConverter.cs

@@ -203,6 +203,11 @@ static List<T> GetEnumList<T>(IEnumerable<string> values)
             type = underlyingType;
         }
 
+        if (value is null)
+        {
+            throw new($"Expected value to be convertible to {type}. However it was null. It is possible a required variable was not provided");
+        }
+
         if (type == typeof(DateTime))
         {
             return ValueConverter.ConvertTo<DateTime>(value);
@@ -225,12 +230,12 @@ static List<T> GetEnumList<T>(IEnumerable<string> values)
 
         if (type == typeof(Guid))
         {
-            return new Guid(value!);
+            return new Guid(value);
         }
 
         if (type.IsEnum)
         {
-            return Enum.Parse(type, value!, true);
+            return Enum.Parse(type, value, true);
         }
 
         return Convert.ChangeType(value, type);