Warning: Invalid argument for foreach() ..\class-wp-list-util.php on line 165

Warning: Invalid argument supplied for foreach() in D:\home\site\wwwroot\wp-includes\class-wp-list-util.php on line 165

I’m getting this warning in the header area of my site. The warning is inconsistent on the pages of the site.
I’m running this on a WordPress network/multisite. No other users are reporting this warning appearing. Turning off the theme seems to rid of the warning but as I mentioned, it’s inconsistent.

	/**
	 * Plucks a certain field out of each element in the input array.
	 *
	 * This has the same functionality and prototype of
	 * array_column() (PHP 5.5) but also supports objects.
	 *
	 * @since 4.7.0
	 *
	 * @param int|string $field     Field to fetch from the object or array.
	 * @param int|string $index_key Optional. Field from the element to use as keys for the new array.
	 *                              Default null.
	 * @return array Array of found values. If <code>$index_key</code> is set, an array of found values with keys
	 *               corresponding to <code>$index_key</code>. If <code>$index_key</code> is null, array keys from the original
	 *               <code>$list</code> will be preserved in the results.
	 */
	public function pluck( $field, $index_key = null ) {
		$newlist = array();

		if ( ! $index_key ) {
			/*
			 * This is simple. Could at some point wrap array_column()
			 * if we knew we had an array of arrays.
			 */
///Line 165 next
			foreach ( $this->output as $key => $value ) {
				if ( is_object( $value ) ) {
					$newlist[ $key ] = $value->$field;
				} else {
					$newlist[ $key ] = $value[ $field ];
				}
			}

			$this->output = $newlist;

			return $this->output;
		}