NoMethodError in Admin/overviewController#index

[isignin]

I installed the Spree-suppliers extension as described in the instructions. After restarted the server, I get the "NoMethodError in Admin/overviewController#index" error.
The log shows:
NoMethodError (You have a nil object when you didn't expect it! You might have expected an instance of Array. The error occurred while evaluating nil.<<>:
vendor/extensions/suppliers/suppliers_extension.rb:25:in 'add_product_supplier_tab'

I'm using spree 0.9.4. Have also tried the edge version with same error.