I’m getting the same error, I deactivated your plugin PayPal WP Button Manager, and was able to edit my pages.
I just install it and before clicking on the button I saw the legend that it wasn’t tested in my current wordress version 5.0.3–es_ES, but I decided to give it a try.
I created a button, and then tried to go to one of my pages to insert the shortcode and get the error.
The error message clearly mentions the directory of the plugin in a couple of lines:
TypeError: this.activateMode is not a function
….
at s (…/wp-content/plugins/paypal-wp-button-manager/admin/gutenberg/blocks.build.js?ver=1:1:54026)
at Function.ss (…./wp-content/plugins/paypal-wp-button-manager/admin/gutenberg/blocks.build.js?ver=1:1:96431)
Your plugin looks very functional, hope you can fix it