@saguaros
I am experiencing the same issue and error when trying to update the system plugin from 3.0.2 to 3.0.7.
One of the menu items on our main menu has an image. Once I removed the image, the site was working again with 3.0.7. I added the image back then I took a quick look at line 131 of plugins\system\t3\includes\menu\t3bootstrap.tpl.php
I tested changing this
$Item->itemParams->get('menu_text', 1) ?
to this
$item->params->get('menu_text', 1) ?
and the site started working again.
I looked back at that file in 3.0.2 and it has line 131 as $item->params->get('menu_text', 1) ?
So, somewhere along the line it was changed.
Perhaps this information can help fix the issue?