Because in MultiColumnWizard the widget doesn't have a fieldname "strField" the icon-font is not found.
To fix this replace in
src/Widget/IconPicker.php
line 41
$fontPath = $GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['eval']['iconFont'];
with the mutch shorter
$fontPath = $this->iconFont;
Because in MultiColumnWizard the widget doesn't have a fieldname "strField" the icon-font is not found.
To fix this replace in src/Widget/IconPicker.php line 41
$fontPath = $GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['eval']['iconFont'];
with the mutch shorter$fontPath = $this->iconFont;