[wp-trac] [WordPress Trac] #43535: wp-mediaelement.min.js?ver=4.9.4:1 crashes, making the Widgets in customize.php unusable

[WordPress Trac]

#43535: wp-mediaelement.min.js?ver=4.9.4:1 crashes, making the Widgets in
customize.php unusable
--------------------------+-----------------------------
 Reporter:  radual        |      Owner:
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  General       |    Version:  4.9.4
 Severity:  normal        |   Keywords:
  Focuses:  javascript    |
--------------------------+-----------------------------
 When accessing **wp-admin/customize.php** with a live preview, I am unable
 to add menu items or to switch between "Text/Visual" for the text widgets.
 At the first sight, it seems to be a problem with Javascript.

 The console looks like having an error with the media gallery - `Uncaught
 TypeError: Cannot read property 'indexOf' of null` (see screenshot for
 details).
 [[Image(https://cloudup.com/cLbAt2w2Kas)]]

 There is also a second error in the console - which does not seem related
 - but it may prove helpful: `Error parsing header X-XSS-Protection: 1;
 mode=block; report=https://www.google.com/appserve/security-
 bugs/log/youtube: insecure reporting URL for secure page at character
 position 22. The default protections will be applied.`

 I have also disabled live preview with a `die()`, the situation is the
 same - which makes me think the error is more related to the way media
 gallery interprets some part of the content while rendering customize.php.
 Everything works smoothly for **wp-admin/widgets.php** (no Javascript
 errors).

 There may be other sections that are affected, but I can confirm these
 two:
 * Widgets > Footer X > Any text widgets: cannot switch between Visual and
 Text (stays blocked on Visual)
 * Menu > Main Menu > Add items: items are not clickable (and the first one
 is not folded).

 Browser: Chrome 64.0

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Ticket URL: <https://core.trac.wordpress.org/ticket/43535>
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