I’ve found a way to fix this, and I intend to post a pre-release here soon for others to confirm the fix before releasing it as 4.0.3.
But it’s possible that the root cause is actually a mistaken assumption in the RankMath code base. I haven’t confirmed that in their codebase, but I’ve reproduced the problem even without activating this plugin (see below, if you like).
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What follows is some more technical description that may only be helpful to RankMath–or might be entertaining to the curious ??
From my vantage point, the fix I plan to make is a change that happens to work fine for this plugin’s purposes, and also avoids the problem scenario with RankMath. But the same error could conceivably surface again with RankMath in combination with some other theme or plugin in the future.
When this plugin detects that it’s running a recent-enough version of WordPress 5 (so that it has compatible JavaScript libraries–WP 5.4 or later), and the icon chooser is enabled, it declares dependence on the WP Core JavaScript libraries wp-editor and wp-blocks. The icon chooser is enabled when there’s an editor on a post.php or post-new.php page.
One of the editor integration bugs discovered in this plugin’s v4.0.0 is that on pages where plugins or themes add Classic Editor instance to pages that are “Gutenberg Pages” (i.e. pages where all the stuff required to run the Block Editor is also loaded), it wasn’t all playing nicely together. So in 4.0.1 we made a (probably temporary) change to prioritize for the Block Editor. So if is_gutenberg_page() is true, then we set up the icon chooser integration for the Block Editor, but not for the Classic Editor.
If it’s a Block Editor (Gutenberg) page, this plugin (currently) does not enable the icon chooser integration for the Classic Editor. So the current limitation is: you can’t have both integrations on the same page–priority goes to Gutenberg.
In this plugin’s v4.0.2, when the icon chooser is enabled on a recent-enough WP5, then the wp-blocks and wp-editor dependencies are declared, even though–because of the above (probably temporary) limitation, the plugin doesn’t end up using them on a non-Gutenberg page. In other words, when you’re not using the Block Editor–only the Classic Editor–those dependencies end up being loaded even though they aren’t necessary in that case. It’s because they aren’t necessary in our case that I can remove them, which satisfies RankMath when running on a Classic Editor-only page. But it seems that it’s the mere loading of those dependencies–nothing more–that breaks RankMath in the Classic Editor.
Any other theme or plugin out there that happens to declare these dependencies would produce the same conditions that result in this error in RankMath. I confirmed this by completely disabling the Font Awesome plugin, and adding a script that does nothing except printing a console message, and that also depends on wp-editor (that’s the smoking gun, not wp-blocks). Just that minimal test case was enough to produce the same error in the RankMath editor, when using the Classic Editor.
