refactor: initialize ErrorWindow on attach

[javier-godoy]

Also initialize it the first time that getChildren is called (even if it isn't attached) in order to allow reusing the dialog contents in
ErrorView:

ErrorWindowAddon/src/main/java/com/flowingcode/vaadin/addons/errorwindow/ErrorView.java

Lines 44 to 49 in 42449d2

	newErrorWindow(parameter.getCaughtException(), i18n) {
	@Override
	publicvoidclose() {
	UI.getCurrent().getPage().getHistory().back();
	}
	}.getChildren().forEach(this::add);

Close#44