Setting gravity constant lower bound causes crash when finishing a trajectory

[hlinhn]

I am running a locally built version of cartographer (built at 105c034577220268cd28a304a185adbec46b729f) on Linux 5.4.0-73-generic

Upon finishing a newly created trajectory (after sending the first initial pose in localization only mode to cartographer_ros), cartographer sometimes crashes with the following trace:

F0519 10:35:41.948809 12819 problem_impl.cc:698] Parameter block not found: 0x7f90ec05aa20. You must add the parameter block to the problem before you can set a lower bound on one of its components.
[FATAL] [1621391741.949467829][/cartographer/cartographer_node][/var/jenkins_home/workspace/cartographer_ros_focal/ws/src/cartographer_ros/cartographer_ros/cartographer_ros/ros_log_sink.cc:63]: F0519 10:35:41.000000 12819 problem_impl.cc:698] Parameter block not found: 0x7f90ec05aa20. You must add the parameter block to the problem before you can set a lower bound on one of its components.
*** Check failure stack trace: ***
    @     0x7f91173411c3  google::LogMessage::Fail()
    @     0x7f911734625b  google::LogMessage::SendToLog()
    @     0x7f9117340ebf  google::LogMessage::Flush()
    @     0x7f91173416ef  google::LogMessageFatal::~LogMessageFatal()
    @     0x7f91174afddc  ceres::internal::ProblemImpl::SetParameterLowerBound()
    @     0x55ecda889667  cartographer::mapping::optimization::OptimizationProblem3D::Solve()
    @     0x55ecda841ce0  cartographer::mapping::PoseGraph3D::RunOptimization()
    @     0x55ecda8429b1  cartographer::mapping::PoseGraph3D::HandleWorkQueue()
    @     0x55ecda860de2  cartographer::mapping::constraints::ConstraintBuilder3D::RunWhenDoneCallback()
    @     0x55ecda8c6ca5  cartographer::common::Task::Execute()
    @     0x55ecda7f8951  cartographer::common::ThreadPool::DoWork()
    @     0x7f9116973d84  (unknown)
    @     0x7f9116e76609  start_thread
    @     0x7f9116661293  clone

I suspect that it was caused by having only one node in the first non-frozen trajectory (hence this parameter block https://github.com/cartographer-project/cartographer/blob/a54e72ec1ee47b4eb8a23581069abd44008668e9/cartographer/mapping/internal/optimization/optimization_problem_3d.cc#L422-L434 was not added), causing the crash at https://github.com/cartographer-project/cartographer/blob/a54e72ec1ee47b4eb8a23581069abd44008668e9/cartographer/mapping/internal/optimization/optimization_problem_3d.cc#L446

It is a little difficult to reproduce this issue reliably, but setting TRAJECTORY_BUILDER_3D.motion_filter.max_time_seconds to a high number and having the tracking frame not moving while sending the first initial pose in localization only mode seems to help with reproducing it.