refactor: Make the code a little safer: ensure we don't reference past end of file

I have been unable to make the previous implementation fail,
but this seems safer - albeit at the expense of complexity.

Author: claremacrae

src/Commands/ToggleDone.ts

@@ -42,11 +42,16 @@ export const toggleDone = (checking: boolean, editor: Editor, view: MarkdownView
     } else {
         // https://github.com/obsidian-tasks-group/obsidian-tasks/issues/3342
         // If insertion.text is empty, delete the line instead, so we don't leave a trailing end-of-line character around.
-        // This behaves gracefully even if there is no end-of-line character on the last line in the file.
-        const from = { line: lineNumber, ch: 0 };
-        const to = { line: lineNumber + 1, ch: 0 };
-        // console.log(`Deleting line+EOL "${editor.getRange(from, to)}", because insertion.text is empty.`);
-        editor.replaceRange('', from, to);
+        if (lineNumber < editor.lineCount() - 1) {
+            const from = { line: lineNumber, ch: 0 };
+            const to = { line: lineNumber + 1, ch: 0 };
+            // console.log(`Deleting line+EOL "${editor.getRange(from, to)}", because insertion.text is empty.`);
+            editor.replaceRange('', from, to);
+        } else {
+            // There is no end-of-line character on our line, which is the last line in the file.
+            // console.log(`Deleting line "${editor.getLine(lineNumber)}", because insertion.text is empty.`);
+            editor.setLine(lineNumber, insertion.text);
+        }
     }
 
     /* Cursor positions are 0-based for both "line" and "ch" offsets.