[BUG] - Sometimes crash on macOS

[lucasjinreal]

When inputting, sometimes will crash (perhaps with probability of 40%).
input with Chinese specifically.

Any chances to have a test?

Thread 0 Crashed:: (input method 18934 com.apple.inputmethod.SCIM) Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	       0x18c713720 __pthread_kill + 8
1   libsystem_pthread.dylib       	       0x18c74bf70 pthread_kill + 288
2   libsystem_c.dylib             	       0x18c658908 abort + 128
3   FlutterMacOS                  	       0x1075cf718 std::_fl::__throw_out_of_range[abi:v15000](char const*) + 4 (stdexcept:271) [inlined]
4   FlutterMacOS                  	       0x1075cf718 std::_fl::basic_string<char16_t, std::_fl::char_traits<char16_t>, std::_fl::allocator<char16_t>>::__throw_out_of_range[abi:v15000]() const + 4 (string:1809) [inlined]
5   FlutterMacOS                  	       0x1075cf718 std::_fl::basic_string<char16_t, std::_fl::char_traits<char16_t>, std::_fl::allocator<char16_t>>::replace(unsigned long, unsigned long, char16_t const*, unsigned long) + 488 (string:3073)
6   FlutterMacOS                  	       0x1075d072c std::_fl::basic_string<char16_t, std::_fl::char_traits<char16_t>, std::_fl::allocator<char16_t>>::replace(unsigned long, unsigned long, std::_fl::basic_string<char16_t, std::_fl::char_traits<char16_t>, std::_fl::allocator<char16_t>> const&) + 8 (string:3168) [inlined]

[angelosilvestre]

@lucasjinreal Do you have any input that is likely to cause this issue so we could try to reproduce it?

This stack trace suggests this is a Flutter engine bug. Does this bug also happen in Flutter regular text field?

[lucasjinreal]

Am not sure what happened here, but here is the observation:

• if keep typing about 10 minutes, with backspace deleting / bolding / heading etc operation (not pure type English words), the crash probabliti is about 90%;
• hadn't notice on En.

For Chinese replication, I just using Chinese input method on macOS, and typing something like:

《被讨厌的勇气》

生活中存在多种可能性，多的比你想象的要多得多。
只有合群才能沿着既定的路线前进。
不合群的人最害怕的就是没有表达的途径。
让自己成为可以为别人创造价值的人，在持续贡献的过程中找到自己的归属感。
核心观点
课题分离：他人的选择是他们自己的课题，可以把马带到河边，但是没必要强迫其喝水。不要对他人课题进行干涉。
自我接纳：不合群并不意味着失败，不合群人的往往具有反叛精神、自驱力、好奇心，这些都是不为大众所接纳的点。

I know it might hard to repliduce and to debug, but the issues actually exist. Hoping we can investigate and keep updating.
(Perhaps under the hood is a flutter bug, actually flutter really supports CJK bad, at least on Chinese, the font boldness always looks weird)

Hints: for reveal on Chinese, one can just switch to Chinese input method, randomly typing en then press space to choose words, keep typing util crash