I had the same issue myself, even using the latest version from Github, i decided to do a bit of rewriting to get around this changing NivoSliderPanel.php to this:
NivoSliderPanel.php:
<?php
class NivoSliderPanel extends DataObject {
static $db = array (
'Title' => 'Text',
'Tagline' => 'HTMLText'
);
static $has_one = array(
'NivoSliderPage' => 'NivoSliderPage',
'PanelImage' => 'Image',
'PanelLink' => 'SiteTree'
);
public function getCMSFields_forPopup(){
$tree=class_exists('SimpleTreeDropdownField')?'SimpleTreeDropdownField':'HTMLDropdownField';
return new FieldSet(
new TextField('Title',_t('SiteTree.METATITLE')),
new ImageField('PanelImage'),
new SimpleTinyMCEField('Tagline',_t('SiteTree.METADESC')),
new $tree("PanelLinkID",_t('RedirectorPage.REDIRECTTOPAGE'),"SiteTree")
);
}
function Thumbnail() {
if ($Image = $this->PanelImage()) {
return $Image->CMSThumbnail();
} else {
return null;
}
}
public function canCreate() {
$member = Member::currentUser();
if($member->inGroup(1)) return true;
else return false;
}
public function canEdit() {
$member = Member::currentUser();
if($member->inGroup(1) || $member->inGroup(2)) return true;
else return false;
}
public function canDelete() {
$member = Member::currentUser();
if($member->inGroup(1)) return true;
else return false;
}
}
Thats cured the issue of uploading, but it will just do one image at a time, which aint so bad in my opinion as you can add your text at the same then move on to adding the next image.