Custom walker with nav icon ( presuming the nav icon is an SVG using file_get_contents() )

custom-walker.php

<?php

// In your functions.php

class Menu_Icon_Walker extends Walker_Nav_Menu {


	function start_lvl( &$output, $depth = 0, $args = array() ) {
		$indent = str_repeat("\t", $depth);
		$output .= "\n$indent\n";
	}

	function end_lvl( &$output, $depth = 0, $args = array() ) {
		$indent = str_repeat("\t", $depth);
		$output .= "$indent\n";
	}

	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$classes[] = 'menu-item-' . $item->ID;

		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
		$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
		$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

		$output .= $indent . '';

		$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
		$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
		$attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
		$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

		// NOTE Update the custom fields id
		$menu_icon = get_field('tps_menu_icon', $item);
		$item_output_icon = '';
		if ( $menu_icon ) {
			$item_output_icon = file_get_contents( $menu_icon );
		}

		$item_output = '<li class="' .  implode(" ", $item->classes) . '">';
			$item_output .= $args->before;
			$item_output .= '<a'. $attributes .'>';
				$item_output .= $item_output_icon;				
				$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
			$item_output .= '</a>';
			$item_output .= $args->after;
		$item_output .= '</li>';

		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}


	function end_el( &$output, $item, $depth = 0, $args = array() ) {
		$output .= "\n";
	}
}