After upgrade no theme

Good evening,

Today I upgraded our store from 1.5.1.3 to 3.0.2.0. After having solved some issues in the config.php files I managed to get back into the admin area again. But I can't enter the store (frontend) anymore because when visiting the store's url the message 'Error: A theme has not been assigned to this store!' appears on the screen.
Looking into the store's settings in the admin area I noticed that the dropdown right of the Theme-lable is saying "heading_title". It looks as if our theme somehow isn't recognized. We have only the default theme from what I see under catalog\view\theme.
I am not sure on how to solve this problem. Could anyone point me in the right direction?

I decided to do a fresh clean install of v3. It all seems to work fine now. Thanks.