Finish null-safety in BizInvokerUtilities

fixes b6f1afcce

src/BizHawk.BizInvoke/BizInvokerUtilities.cs

@@ -5,22 +5,30 @@ namespace BizHawk.BizInvoke
 {
 	public static unsafe class BizInvokerUtilities
 	{
-#nullable disable //TODO If U1 and U2 were structs, this wouldn't be a problem. Would changing them to structs break this helper's functionality? --yoshi
 		[StructLayout(LayoutKind.Explicit)]
 		private class U
 		{
 			[FieldOffset(0)]
-			public U1 First;
+			public readonly U1? First;
+
 			[FieldOffset(0)]
-			public U2 Second;
+			public readonly U2 Second;
+
+			public U(U2 second)
+			{
+				// being a union type, these assignments affect the value of both fields so they need to be in this order
+				First = null;
+				Second = second;
+			}
 		}
-#nullable restore
+
 		[StructLayout(LayoutKind.Explicit)]
 		private class U1
 		{
 			[FieldOffset(0)]
 			public UIntPtr P;
 		}
+
 		[StructLayout(LayoutKind.Explicit)]
 		private class U2
 		{
@@ -29,10 +37,12 @@ namespace BizHawk.BizInvoke
 
 			public U2(object target) => Target = target;
 		}
+
 		private class CF
 		{
 			public int FirstField;
 		}
+
 		/// <summary>
 		/// Didn't I have code somewhere else to do this already?
 		/// </summary>
@@ -43,19 +53,20 @@ namespace BizHawk.BizInvoke
 			int ret;
 			fixed(int* fx = &c.FirstField)
 			{
-				var u = new U { Second = new U2(c) };
-				ret = (int)((ulong)(UIntPtr)fx - (ulong)u.First.P);
+				U u = new(new U2(c));
+				ret = (int) ((ulong) (UIntPtr) fx - (ulong) u.First!.P);
 			}
 			return ret;
 		}
+
 		public static int ComputeStringOffset()
 		{
 			var s = new string(new char[0]);
 			int ret;
 			fixed(char* fx = s)
 			{
-				var u = new U { Second = new U2(s) };
-				ret = (int)((ulong)(UIntPtr)fx - (ulong)u.First.P);
+				U u = new(new U2(s));
+				ret = (int) ((ulong) (UIntPtr) fx - (ulong) u.First!.P);
 			}
 			return ret;
 		}