Using useSelect creates “minified React error”

If you check the URL given with the error message, you will see the full text as:

Rendered more hooks than during the previous render.

Inspecting the code, I see two uses of hooks:

export default function Edit() {
    const allCPTposts = useSelect(select => {
//                      ^ first hook use 
        return select("core").getEntityRecords("postType", "post", {per_page: -1})

    if(allCPTposts == undefined){
        return <p>Loading Posts...</p>

    return (
        <p { ...useBlockProps() }>
{/*             ^ second hook use */}
            { __( 'Dropdown Menu – hello from the editor!', 'dropdown' ) }

So it seems sometimes just useSelect() and sometimes both useSelect() and useBlockProps() are called.

Seeing that there is an early return before the second hook use, this is most likely the issue here. Just move the useBlockProps() call above the early return and the component will always call the same amount of hooks for each render.