# Skip whitespace characters (if any) then return the following non-whitespace character.
	#
	# Returns the code point of the character.
	# Returns `null` on end of file or error.
	#
	# In fact, this method works like `read_char` except it skips whitespace.
	#
	# ~~~
	# var w = new StringReader(" \nab\tc")
	# assert w.read_nonwhitespace == 'a'
	# assert w.read_nonwhitespace == 'b'
	# assert w.read_nonwhitespace == 'c'
	# assert w.read_nonwhitespace == null
	# ~~~
	#
	# `Char::is_whitespace` determines what is a whitespace.
	fun read_nonwhitespace: nullable Char
	do
		var c: nullable Char = null
		while not eof do
			c = read_char
			if c == null or not c.is_whitespace then break
		end
		return c
	end