wwwImage doesnt show

[Mooike]

Hello,
Im using this cordova plugin with capacitor. Everything works fine but when I want to display some wwwImage in my button it doesnt show.
My Implementation looks like that:
backButton: { wwwImage: 'assets/img/back.png', wwwImageDensity: 2, align: 'left', event: 'backPressed' }
After debugging I get the attached error.

Is this a bug or do I make any mistakes ?

[t8skono]

Edit here.
master...jpiar:cordova-plugin-themeablebrowser:master#diff-1d62b12587c770622bad767b84d575bbacf02bad3b1fe65e3bb390dc5fca4672L1017