LazyInitializer.EnsureInitialized does not allow null as return value. Use LazyInit instead.

ICSharpCode.ILSpyX/LoadedAssembly.cs

@@ -209,14 +209,19 @@ namespace ICSharpCode.ILSpyX
 		/// </remarks>
 		public ICompilation? GetTypeSystemOrNull()
 		{
-			return LazyInitializer.EnsureInitialized(ref this.typeSystem, () => {
+			var value = Volatile.Read(ref this.typeSystem);
+			if (value == null)
+			{
 				var module = GetMetadataFileOrNull();
 				if (module == null || module.IsMetadataOnly)
-					return null!;
+					return null;
-				return new SimpleCompilation(
+				value = new SimpleCompilation(
 					module.WithOptions(TypeSystemOptions.Default | TypeSystemOptions.Uncached | TypeSystemOptions.KeepModifiers),
 					MinimalCorlib.Instance);
-			});
+				value = LazyInit.GetOrSet(ref this.typeSystem, value);
+			}
+
+			return value;
 		}
 
 		readonly object typeSystemWithOptionsLockObj = new object();