Customizer not able to commit changes

When the companion plugin is enabled, saving changes in Customizer won’t work (infinite spinner)

Console throws the following error:

customizer-base.js?ver=1.0.134:999 Uncaught TypeError: Cannot read property 'phpTrim' of undefined
    at Object.setMod (customizer-base.js?ver=1.0.134:999)
    at customizer-base.js?ver=1.0.134:1195
    at Function.m.each.m.forEach (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,underscore,customize-base,wp-a11y,wp-util,jquery-ui-core,customize-controls,jquery-ui-widget,jquery-u&load[]=i-position,jquery-ui-tooltip,jquery-ui-mouse,jquery-ui-sortable,jquery-ui-draggable,jquery-ui-droppable,backbone,wp-backbone,wp-&load[]=ajax-response,jquery-color,wp-lists,postbox,nav-menu,utils,media-models,moxiejs,plupload,wp-plupload&ver=4.9:14)
    at Object.setMultipleMods (customizer-base.js?ver=1.0.134:1194)
    at Object._setContent (customizer-base.js?ver=1.0.134:1317)
    at Object.<anonymous> (customizer-base.js?ver=1.0.134:1322)
    at c (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,underscore,customize-base,wp-a11y,wp-util,jquery-ui-core,customize-controls,jquery-ui-widget,jquery-u&load[]=i-position,jquery-ui-tooltip,jquery-ui-mouse,jquery-ui-sortable,jquery-ui-draggable,jquery-ui-droppable,backbone,wp-backbone,wp-&load[]=ajax-response,jquery-color,wp-lists,postbox,nav-menu,utils,media-models,moxiejs,plupload,wp-plupload&ver=4.9:14)