The "ultimate-hop-popping" term refers to the action taken by the penultimate router in an LSP to remove the MPLS label before delivering the packet to the ultimate router,which is not desired here. Configuring the
"explicit-null" statement causes the penultimate router to replace the top label with a label that has a value of
0, which instructs the ultimate router to perform a label lookup and preserve the label switching for the entire LSP.
References:
Juniper Networks documentation on MPLS: MPLS Applications User Guide for Routing Devices

To configure filter-based forwarding, perform the following tasks:
- Create a match filter on the ingress device. To specify a match filter, include the filter filter-name statement at the [edit firewall] hierarchy level. A packet that passes through the filter is compared against a set of rules to classify it and to determine its membership in a set. Once classified, the packet is forwarded to a routing table specified in the accept action in the filter description language. The routing table then forwards the packet to the next hop that corresponds to the destination address entry in the table.
- Create routing instances that specify the routing table(s) to which a packet is forwarded, and the destination to which the packet is forwarded at the [edit routing- instances] hierarchy level.