show N64 menu in null emulator mode (no rom loaded), single instance mode - message box alerting the user that a reboot is required

BizHawk.MultiClient/MainForm.MenuItems.cs

@@ -1184,6 +1184,8 @@ namespace BizHawk.MultiClient
 		private void singleInstanceModeToolStripMenuItem_Click(object sender, EventArgs e)
 		{
 			Global.Config.SingleInstanceMode ^= true;
+
+			MessageBox.Show("BizHawk must be restarted for this setting to take effect.", "Reboot Required", MessageBoxButtons.OK, MessageBoxIcon.Information);
 		}
 
 		private void MainForm_Deactivate(object sender, EventArgs e)

BizHawk.MultiClient/MainForm.cs

@@ -815,6 +815,10 @@ namespace BizHawk.MultiClient
 
 			switch (system)
 			{
+				default:
+				case "NULL":
+					n64ToolStripMenuItem.Visible = true;
+					break;
 				case "TI83":
 					tI83ToolStripMenuItem.Visible = true;
 					break;