Reduced with m-c:
BuildID=20190819150944
SourceStamp=1868b438bd8307c67a4b3f569239410b843722e0

Hit MOZ_CRASH(called Option::unwrap() on a None value) at src/libcore/option.rs:347

#0 MOZ_Crash(char const*, int, char const*) src/obj-firefox/dist/include/mozilla/Assertions.h:313:3
#1 GeckoCrash src/toolkit/xre/nsAppRunner.cpp:5124
#2 gkrust_shared::panic_hook::h441520810dd9ad98 src/toolkit/library/rust/shared/lib.rs:248:8
#3 core::ops::function::Fn::call::hc5a947b91353bd68 /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libcore/ops/function.rs:69:4
#4 std::panicking::rust_panic_with_hook::h096dff2cb12b67ce /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/panicking.rs:481:16
#5 std::panicking::continue_panic_fmt::h0046167652be99a6 /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/panicking.rs:384:4
#6 rust_begin_unwind /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/panicking.rs:311:4
#7 core::panicking::panic_fmt::h8980489a6c4a44cb /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libcore/panicking.rs:85:13
#8 core::panicking::panic::hedd9888e25456fbe /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libcore/panicking.rs:49:4
#9 webrender::util::project_rect::ha3b8733ef000b4bb src/gfx/wr/webrender/src/util.rs
#10 webrender::prim_store::SpaceMapper$LT$F$C$T$GT$::map::hf56ecd10c59a8ee7 src/gfx/wr/webrender/src/prim_store/mod.rs:237
#11 webrender::picture::PicturePrimitive::post_update::h6a5bb9aece41c682 src/gfx/wr/webrender/src/picture.rs:3395:47
#12 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1753
#13 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#14 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#15 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#16 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#17 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#18 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#19 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#20 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#21 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#22 webrender::picture::PictureUpdateState::update::h50e36fd3a321d6bb (.llvm.10953147888855500639) src/gfx/wr/webrender/src/picture.rs:1743:16
#23 webrender::picture::PictureUpdateState::update_all::hba7f3912eb7cbebc src/gfx/wr/webrender/src/picture.rs:1667:8
#24 webrender::frame_builder::FrameBuilder::build_layer_screen_rects_and_cull_layers::h5ab3867fbe1ff318 src/gfx/wr/webrender/src/frame_builder.rs:380:8
#25 webrender::frame_builder::FrameBuilder::build::h08cedb224a5b8d6f src/gfx/wr/webrender/src/frame_builder.rs:553
#26 webrender::render_backend::Document::build_frame::hf1c20cb6fa11e4f2 src/gfx/wr/webrender/src/render_backend.rs:541:24
#27 webrender::render_backend::RenderBackend::update_document::ha73afd95e0faf714 src/gfx/wr/webrender/src/render_backend.rs:1504:40
#28 webrender::render_backend::RenderBackend::prepare_transactions::hc1b4ac4df160f59c src/gfx/wr/webrender/src/render_backend.rs:1331:16
#29 webrender::render_backend::RenderBackend::process_api_msg::h53c63bb00e85350f src/gfx/wr/webrender/src/render_backend.rs:1223
#30 webrender::render_backend::RenderBackend::run::h1bc67c7feeb12bfd src/gfx/wr/webrender/src/render_backend.rs:974:20
#31 webrender::renderer::Renderer::new::_$u7b$$u7b$closure$u7d$$u7d$::hb9061a1e51cca689 src/gfx/wr/webrender/src/renderer.rs:2198:12
#32 std::sys_common::backtrace::__rust_begin_short_backtrace::he6e57ecf74f5064d /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/sys_common/backtrace.rs:77
#33 std::thread::Builder::spawn_unchecked::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::h0e72aea74b04c6fb /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/thread/mod.rs:470:16
#34 _$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$$LP$$RP$$GT$$GT$::call_once::h710d93ff86661e0e /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/panic.rs:315
#35 std::panicking::try::do_call::h555fb81df29409af (.llvm.15886174702357668173) /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libstd/panicking.rs:296
#36 __rust_maybe_catch_panic /rustc/61d1607e0f6a18bb4897d6f9b10abeac9e11eb8e/src/libpanic_abort/lib.rs:29:4

I get a crash like this on the testcase:
https://crash-stats.mozilla.org/report/index/b54c2e87-b23e-4e94-b910-4893a0190821#tab-bugzilla

cant repro anymore.
Maybe fixed by bug 1562869 ?

(In reply to Mayank Bansal from comment #2)

Maybe fixed by bug 1562869 ?

This was last seen by the fuzzers in Aug 2019. So that seems reasonable.