ContainerSciBench.scibench;
createNewItem(GuiSciBench.elements)
There's not much code to go off, but...
What are you accessing here? are those static fields?
Your gui and container should be normal classes, there can and will be multiple instances of them at the same time. In SP the container will exist in 2 copies, one for the client side and one for the server, make sure you are working with the right one.
Also your Gui class only exists on the client side, never on the server, while anything that actually changes slots should happen on the server, this looks fishy.