This is an issue with Visual Composer. They are failing to do a sanity check to ensure that the array is not returning empty. You’ll notice the error path comes from …plugins/js_composer… not from ACF’s plugin. The reason deactivating it worked is because VC was no longer looking for anything from ACF.
More to the point, perusing the code it appears you may have a field group defined in ACF that actually has no fields within it. Which would cause that error. However I can’t tell for sure without knowing your setup.
