fix(reply): Avoid crash due to negative range start value in reply (#528)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: jstoffan

src/components/ReplyField/ReplyField.tsx

@@ -81,7 +81,7 @@ export default class ReplyField extends React.Component<Props, State> {
 
         const { mentionString, mentionTrigger } = activeMention;
         const offset = mentionString.length + mentionTrigger.length;
-        const mentionStart = currentCursor - offset;
+        const mentionStart = Math.max(0, currentCursor - offset);
 
         range.setStart(textNode, mentionStart);
         range.setEnd(textNode, mentionStart);