Send an ErrorMessage to the client if they try to draw a line before selecting a plane

Nibriboard/Client/Messages/ErrorMessage.cs

@@ -5,12 +5,12 @@ namespace Nibriboard.Client.Messages
 	/// Represents a complaint (usually by the server) about something that the other party
 	/// has done that probably shouldn't have.
 	/// </summary>
-	public class ErrorMessage
+	public class ErrorMessage : Message
 	{
 		/// <summary>
 		/// The message describing the error that has occurred.
 		/// </summary>
-		string Message;
+		public string Message;
 
 		public ErrorMessage()
 		{

Nibriboard/Client/NibriClient.cs

@@ -375,6 +375,14 @@ namespace Nibriboard.Client
 			line.LineWidth = message.LineWidth;
 			line.Colour = message.LineColour;
 
+			if(CurrentPlane == null)
+			{
+				Send(new ErrorMessage() {
+					Message = "Error: You can't complete a line until you've selected a plane" +
+						"to draw it on!"
+				});
+				return;
+			}
 			await CurrentPlane.AddLine(line);
 		}