From Documentation
In the following example, you can see the lifecycle difference of EL and zscript.
<window>
<button label="${btn2.label}" id="btn1" onClick='btn2.label="changed"'/>
<button label="ok" id="btn2"/>
</window>
In component creation phase, component is evaluated by sequence in file, it reserves XML tree structure, parent is always evaluated before children. When label="${btn2.label}" is evaluated, btn2 is not evaluated yet, therefore it gets null. zscript is evaluated at rendering phase, btn2 is already created, therefore it knows how to find btn2