Allows you to enable and disable the wizard create button, so that the user can not click it.

	// C#
	// Asks the user to set the var "Number" to 5, if is not set to 5
	// the "Finish" button will not be reachable
	using UnityEngine;
	using UnityEditor;
		
	public class isValidScriptableWizard : ScriptableWizard {
		public int number = 0;
		[MenuItem ("Example/Show isValid Usage")]
		static void CreateWindow() {
			ScriptableWizard.DisplayWizard(
				"isValid boolean example", 
				typeof(isValidScriptableWizard), 
				"Finish");
		}
		void OnWizardUpdate() {
			helpString = "Set The number to 5 and press finish";
			if(number != 5) {
				errorString = "The number has to be set to 5!";
				isValid = false;
			} else { 
				errorString = "";
				isValid = true;
			}
		}
	}