Your installation can not find any theme plugins. It tries that here, but that call is returning null in your installation: pkp-lib/PKPTemplateManager.inc.php at ojs-stable-3_1_1 · pkp/pkp-lib · GitHub
What do you have in the plugins/themes/ folder? And if there are subfolders, do they have content in them?
In Settings > Website > Plugins, do you you see theme plugins and are they enabled?
In Settings > Website > Appearance what plugin is activated?
edit: also check this thread and the solution in the bottom: [OJS 3.1.1] Create Journal loads indefinitely, journal db exists, but all Javascript missing - #12 by Oanh_Hoang