check_theme_switched() will Fatal Error if the old theme cannot be found for some reason

[westi]

In check_theme_switched() we assume that we can get a WP_Theme object for the old theme.

If this fails because the theme no longer exists or the option data doesn't match an available theme then we try and call a method on an object which doesn't exist.

As I said to westi in IRC, the theme_switched option is only supposed to exist until the next pageload. So if it still exists, then you have bigger problems. I'm definitely fine with checking the return value of wp_get_theme() and using the raw $stylesheet if necessary.

In [20330]:

Avoid fatal errors in check_theme_switched() if we fail to get a WP_Theme object for the old theme. Fixes #20334.

[20363]

This is still not completely fixed. Easiest way to duplicate is by renaming a theme in it's style.css when you're already at wp-admin/themes.php.

• Visit wp-admin/themes.php
• Rename any theme in style.css
• Refresh wp-admin/themes.php
• Fatal error: Call to a member function exists() on a non-object in /wp.local/wp/wp-includes/theme.php on line 1557
• Error points to check_theme_switched()

Patch below circumvents the problem without digging too deep into the API.

This might have been fixed in [20524].

Confirmed fixed. The steps above no longer produce any errors.

Leaving this open to investigate a possible bug in the check_theme_switched() implementation.