The following is the run window output:

This text will be displayed in red
Finished with error: Bad UTF-8 encoding (U+FFFD; REPLACEMENT CHARACTER) found while decoding string: Text before and after the index: >>>nschauen. Bitte nicht lachen �<<<
. The Flutter team would greatly appreciate if you could file a bug explaining exactly what you were doing when this happened:
https://github.com/flutter/flutter/issues/new/choose
The source bytes were:
[84, 101, 120, 116, 32, 98, 101, 102, 111, 114, 101, 32, 97, 110, 100, 32, 97, 102, 116, 101, 114, 32, 116, 104, 101, 32, 105, 110, 100, 101, 120, 58, 32, 62, 62, 62, 110, 115, 99, 104, 97, 117, 101, 110, 46, 32, 66, 105, 116, 116, 101, 32, 110, 105, 99, 104, 116, 32, 108, 97, 99, 104, 101, 110, 32, 239, 191, 189, 60, 60, 60, 10]

I was selecting random words (tokens) in a Corpus (String variable, containing a chat conversation)

The excerpt above:
"Text before and after the index: >>>nschauen. Bitte nicht lachen �<<<"
is created with the following print statement:
print("Text before and after the index: >>>${_text.substring(_indexOfWordInOriginalTextBody - 10, _indexOfWordInOriginalTextBody+20)}<<<");

The original line looks like this:
"Essen Gilmore Girls anschauen. Bitte nicht lachen 😜"