AJAX "Fatal error: [] operator not supported for strings" when adding fields to new display with aggregation enabled

Ran into this too. What's the best way to fix? Is it simple enough to pass the variable as a reference instead?

I receive this error when trying add a new taxonomy filter on a view with aggregation. Any ideas?

I have exact same issue as #3

issue also occurs in php7

in my case we were adding a term reference field.
only had Fatal error: [] operator not supported for strings in modules/views/includes/admin.inc on line 2399

IMO: major. this stops anyone from being able to build a View.

short term work around: turn of javascript on your browser and build your view.

Even-though #2823977: Ajax http 500 error using taxonomy to filter view was filed later than this one, it has already working patch for the same problem, so I am marking this as duplicate.

Though this is closed, I still found this issue when going through google. I saw the first error, about how you can't use [] operand on string, after updating to PHP7.0.

The other patch mentioned in comment #7 didn't work for me.

I made my own edits to views/include/admin.inc. Starting on line 2391, this is the entire views_ui_regenerate_tab function, including my change:

function views_ui_regenerate_tab(&$view, &$output, $display_id) {
  if (!$view->set_display('default')) {
    return;
  }

  ///////////////////////////
  // Added by RP on 3.20.2018, to fix bug where view kept showing error about how
  // you couldn't use [] on a String.  It's the $output variable.  
  // basically, just make sure the $output var is an array.  If it had a string in it originally,
  // then let's add that in as its first element.
  if (!is_array($output)) {
    $original = $output;
    $output = array();
    if ($original != "") {
      $output[] = $original;
    }
  }
  //////////////////////////////  

  // Regenerate the main display area.
  $build = views_ui_get_display_tab($view, $display_id);
  views_ui_add_microweights($build);
  $output[] = ajax_command_html('#views-tab-' . $display_id, drupal_render($build));

  // Regenerate the top area so changes to display names and order will appear.
  $build = views_ui_render_display_top($view, $display_id);
  views_ui_add_microweights($build);
  $output[] = ajax_command_replace('#views-display-top', drupal_render($build));
}

Ok, lets try this then.

At line 2497 is_array() is missing the parameter, I believe it should be
if (!is_array($output)) {

Now adding a field does nothing in the Views UI and an error is logged:
Warning: is_array() expects exactly 1 parameter, 0 given in views_ui_regenerate_tab() (line 2497 of ..../sites/all/modules/contrib/views/includes/admin.inc).

Here's an updated patch that contains the recommended fix from #11.

I've also filed a similar issue against Backdrop core:
https://github.com/backdrop/backdrop-issues/issues/3244

Patch #12 fixed the issue on my websites. :)

The patch in #12 to admin.inc does not apply to the latest views 7.x-3.x-dev.

Checking patch includes/admin.inc...
error: while searching for:
  array_splice($args, 0, 4);

  $form_state = views_ui_build_form_state($js, $key, $view, $display_id, $args);
  // check to see if this is the top form of the stack. If it is, pop
  // it off; if it isn't, the user clicked somewhere else and the stack is
  // now irrelevant.
  if (!empty($view->stack)) {

error: patch failed: includes/admin.inc:3004
error: includes/admin.inc: patch does not apply

A patch with just the meat, and no typo fixes in the comments.

Patch rerolled.

1. The typehint in the function signature causes a type error when the $output variable is a string.
2. $output is expected to be an array of ajax commands. This doesn't seem like a proper solution.