TypeError: Cannot read properties of undefined (reading ‘trim’)

how u doing?

I using this PHP validation below to send a http response to a javascript function, but I’m getting the error “TypeError: Cannot read properties of undefined (reading ‘trim’)”.

Can anyone help me with to identify where should a change to fix this typeerror?

PHP

    if(!$validate_status) {
      header($_SERVER["SERVER_PROTOCOL"]." 400 Bad Request");
    } else {
      header($_SERVER["SERVER_PROTOCOL"]." 200 OK");
    }

Javascript

function validate_submit(thisForm, action, formData) {
    fetch(action, {
      method: 'POST',
      body: formData,
      headers: {'X-Requested-With': 'XMLHttpRequest'}
    })
    .then(response => {
      if( response.ok ) {
        thisForm.querySelector('.loading').classList.remove('d-block');
        thisForm.querySelector('.sent-message').classList.add('d-block');
        thisForm.reset();         
      } else {
        throw new Error(`${response.status} ${response.statusText} ${response.url}`); 
      }
    })
    .then(data => {
      thisForm.querySelector('.loading').classList.remove('d-block');
      if (data.trim() == 'OK') {
        thisForm.querySelector('.sent-message').classList.add('d-block');
        thisForm.reset(); 
      } else {
        throw new Error(data ? data : 'Form submission failed and no error message returned from: ' + action);
      }
    })
    .catch((error) => {
      displayError(thisForm, error);
    });
  }

I tried to debug with chrome dev tools, but with no success. I couldn’t find out why data content sent from php is undefined and I don’t know how to fix it.