assertion failed: !self.nodes.contains_key(&id) (thread RenderBackend, at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/clip_scroll_tree.rs:493)
stack backtrace:
   0:     0x55b6a85741b4 - backtrace::backtrace::libunwind::trace
                        at .cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/libunwind.rs:53
                         - backtrace::backtrace::trace<closure>
                        at .cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/backtrace/mod.rs:42
   1:     0x55b6a856950f - backtrace::capture::{{impl}}::new
                        at .cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.2/src/capture.rs:64
   2:     0x55b6a7a725de - servo::main::{{closure}}
                        at ports/servo/main.rs:130
   3:     0x55b6ae2b7556 - std::panicking::rust_panic_with_hook
                        at /checkout/src/libstd/panicking.rs:578
   4:     0x55b6a8b85d96 - std::panicking::begin_panic<&str>
                        at /checkout/src/libstd/panicking.rs:538
   5:     0x55b6a89126f6 - webrender::clip_scroll_tree::{{impl}}::add_node
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/clip_scroll_tree.rs:493
   6:     0x55b6a8b3e7d3 - webrender::frame_builder::{{impl}}::add_scroll_frame
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame_builder.rs:502
   7:     0x55b6a8b34f4e - webrender::frame::{{impl}}::flatten_scroll_frame
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:197
   8:     0x55b6a8b3795a - webrender::frame::{{impl}}::flatten_item
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:586
   9:     0x55b6a8b3499c - webrender::frame::{{impl}}::flatten_items
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:151
  10:     0x55b6a8b35827 - webrender::frame::{{impl}}::flatten_stacking_context
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:294
  11:     0x55b6a8b374c2 - webrender::frame::{{impl}}::flatten_item
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:534
  12:     0x55b6a8b3499c - webrender::frame::{{impl}}::flatten_items
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:151
  13:     0x55b6a8b34675 - webrender::frame::{{impl}}::flatten_root
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:117
  14:     0x55b6a8b3aa13 - webrender::frame::{{impl}}::create
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/frame.rs:1121
  15:     0x55b6a89f3652 - webrender::render_backend::{{impl}}::build_scene
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/render_backend.rs:89
  16:     0x55b6a89f4c34 - webrender::render_backend::{{impl}}::process_document
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/render_backend.rs:293
  17:     0x55b6a89f60f5 - webrender::render_backend::{{impl}}::run
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/render_backend.rs:502
  18:     0x55b6a8958425 - webrender::renderer::{{impl}}::new::{{closure}}
                        at .cargo/git/checkouts/webrender-c3596abe1cf4f320/aa74cdf/webrender/src/renderer.rs:1815
  19:     0x55b6a889b05a - std::sys_common::backtrace::__rust_begin_short_backtrace<closure,()>
                        at /checkout/src/libstd/sys_common/backtrace.rs:134
  20:     0x55b6a8a89cb3 - std::thread::{{impl}}::spawn::{{closure}}::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:400
  21:     0x55b6a88842ca - std::panic::{{impl}}::call_once<(),closure>
                        at /checkout/src/libstd/panic.rs:296
  22:     0x55b6a8b870d2 - std::panicking::try::do_call<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panicking.rs:480
  23:     0x55b6ae2be69c - panic_unwind::__rust_maybe_catch_panic
                        at /checkout/src/libpanic_unwind/lib.rs:99
  24:     0x55b6a8b862a2 - std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
                        at /checkout/src/libstd/panicking.rs:459
  25:     0x55b6a889c235 - std::panic::catch_unwind<std::panic::AssertUnwindSafe<closure>,()>
                        at /checkout/src/libstd/panic.rs:361
  26:     0x55b6a8a89ad1 - std::thread::{{impl}}::spawn::{{closure}}<closure,()>
                        at /checkout/src/libstd/thread/mod.rs:399
  27:     0x55b6a8ab4dc3 - alloc::boxed::{{impl}}::call_box<(),closure>
                        at /checkout/src/liballoc/boxed.rs:762
  28:     0x55b6ae2b626b - alloc::boxed::{{impl}}::call_once<(),()>
                        at /checkout/src/liballoc/boxed.rs:772
                         - std::sys_common::thread::start_thread
                        at /checkout/src/libstd/sys_common/thread.rs:24
                         - std::sys::imp::thread::{{impl}}::new::thread_start
                        at /checkout/src/libstd/sys/unix/thread.rs:90
  29:     0x7fa5e732c493 - start_thread
  30:     0x7fa5e6c51abe - __clone
  31:                0x0 - <unknown>
ERROR:servo: assertion failed: !self.nodes.contains_key(&id)