The error is because the plugin is loading twice with page and in a popover dialog, and in the popover dialog the Handlebar js is not being loaded that causing the issue, in case if you are using custom plugins then add below condition in plugin js to load plugins only once.
if (document.location.pathname.indexOf("/editor.html/") > -1)
addDialogTemplate();
I've updated the color picker and created new custom style plugin and both are working fine in 6.3,6.4 and 6.5