Update runtime files.

runtime/doc/vim9class.txt

@@ -1,4 +1,4 @@
-*vim9class.txt*	For Vim version 9.0.  Last change: 2023 Jan 17
+*vim9class.txt*	For Vim version 9.0.  Last change: 2023 Feb 19
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -471,14 +471,14 @@ Items in a class ~
 						*E1318* *E1325* *E1326*
 Inside a class, in betweeen `:class` and `:endclass`, these items can appear:
 - An object member declaration: >
-  	this._memberName: memberType
-  	this.memberName: memberType
+	this._memberName: memberType
+	this.memberName: memberType
 	public this.memberName: memberType
 - A constructor method: >
-  	def new(arguments)
-  	def newName(arguments)
+	def new(arguments)
+	def newName(arguments)
 - An object method: >
-  	def SomeMethod(arguments)
+	def SomeMethod(arguments)
 <							*E1329*
 For the object member the type must be specified.  The best way is to do this
 explicitly with ": {type}".  For simple types you can also use an initializer,
@@ -520,6 +520,15 @@ name, with a hint about what it provides.
 An interface can only be defined in a |Vim9| script file.  *E1342*
 
 
+null object ~
+
+When a variable is decleared to have the type of an object, but it is not
+initialized, the value is null.  When trying to use this null object Vim often
+does not know what class was supposed to be used.  Vim then cannot check if
+a member name is correct and you will get an "Using a null object" error,
+even when the member name is invalid. *E1360*
+
+
 Default constructor ~
 
 In case you define a class without a new() method, one will be automatically