get max_alt_level failed

[Xudong-Huang]

thread '' panicked at 'called Result::unwrap() on an Err value: InvalidColumnType(0, Null)', libcore\result.rs:983:5 stack backtrace: 0: std::sys::windows::backtrace::set_frames at C:\projects\rust\src\libstd\sys\windows\backtrace\mod.rs:104 1: std::sys::windows::backtrace::set_frames ...... 25: alloc::vec::{{impl}}::from_iter<u32,mut core::result::{{impl}}::from_iter::Adapter<rusqlite::row::MappedRows, rusqlite::error::Error>*> at C:\projects\rust\src\liballoc\vec.rs:1772 26: core::result::{{impl}}::from_iter<u32,rusqlite::error::Error,alloc::vec::Vec,rusqlite::row::MappedRows> at C:\projects\rust\src\libcore\result.rs:1224 27: core::iter::iterator::Iterator::collect<rusqlite::row::MappedRows,core::result::Result<alloc::vec::Vec, rusqlite::error::Error>> at C:\projects\rust\src\libcore\iter\iterator.rs:1415 28: trustnote::main_chain::update_stable_mc_flag at D:\project\rust\rust-trustnote\src\main_chain.rs:958 29: trustnote::main_chain::update_main_chain at D:\project\rust\rust-trustnote\src\main_chain.rs:1119 30: trustnote::joint::Joint::save at D:\project\rust\rust-trustnote\src\joint.rs:625 31: trustnote::network::network::WsConnection::handle_online_joint at D:\project\rust\rust-trustnote\src\network\hub.rs:991 32: trustnote::network::hub::{{impl}}::request_joints::request_joint