How to overcome this WordPress magic?

Question

WordPress

Hello

I'm including the file with the class on the test page - everything works as it should.

I connect the same file with the class through functions.php - it gives errors on the second and fifth lines of this code fragment:

$m_l = $m;
$st_pos = strpos($this->a_u, $m[0]); // Warning:  Uninitialized string offset 0
if ($st_pos !== false) {
    $ll = $this->left_limit_uc;
    $m_l[0] = $this->a[$st_pos]; // Warning:  Trying to access array offset on value of type null
}

What can be wrong?