Connector is not initialized if the grid is already attached

[javier-godoy]

GridHelper fails to initialize the connector if the grid is already attached when the helper object is instantiated.

GridHelpers/src/main/java/com/flowingcode/vaadin/addons/gridhelpers/GridHelper.java

Lines 84 to 90 in a4ad7f3

	private GridHelper(Grid<T> grid) {
	this.grid = grid;
	this.helperClassNameGenerator = new GridHelperClassNameGenerator<>();
	setClassNameGenerator(grid.getClassNameGenerator());
	grid.addItemClickListener(this::onItemClick);
	grid.addAttachListener(this::onAttach);
	}