I am using Mathematica 12 for linux x64. I did a lot work in a mathematica session, including some Parallelsubmit and Waitall. All parallel kernels were closed excepted the main kernel. Then, when I want to insert a Horizontal Thick Line, the Mathematica frontend freezes. In the terminal, it showed

Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion '(lines-1)->lastCharIsLineBreak' failed at LineWrapBox.c:4243", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'i == n-1' failed at LineWrapBox.c:4244", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'lines->lineIsNull()' failed at LineWrapBox.c:4638", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion '(lines-1)->lastCharIsLineBreak' failed at LineWrapBox.c:4243", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'i == n-1' failed at LineWrapBox.c:4244", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'lines->lineIsNull()' failed at LineWrapBox.c:4638", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion '(lines-1)->lastCharIsLineBreak' failed at LineWrapBox.c:4243", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'i == n-1' failed at LineWrapBox.c:4244", and describe in as much detail as possible what you were doing when the error occurred.
Mathematica has detected a possible internal error. If possible, please report the error to support@wolfram.com, quoting "Assertion 'lines->lineIsNull()' failed at LineWrapBox.c:4638", and describe in as much detail as possible what you were doing when the error occurred.

I don't know whether these messages are related to this freezing. Just copy them here.