Validate hidden input existence when saving value in problemgrader.js.

This is the real problem that prevents the functionality of the problem
grader and causes issue openwebwork#2215.  These hidden inputs not existing for
some problems in a test with non-consecutive problems causes a
javascript error that prevents the comment from being saved.  If the
javascript error does not occur then the comment will be saved
correctly.

To test this create a test with problems 1, 2, 3, and 4, and delete
problem 2 (for example).  Then submit a version of the test, and open
the problem grader in that version.  Then go to problem 2 and set the
score and add a comment for that problem.  With the develop branch you
will get the message

    Error saving score.
    document.gwquiz.elements[("probstatus" + s.dataset.problemId)] is undefined

If you check the database, you will see that the score is saved, but the
comment is not.  With this branch the score and comment will be
successfully saved (and you can verify this in the database).

There is still an issue with indexing of the `probstatus` fields in
that needs to be resolved.  See my comment in openwebwork#2216 which gives an
indication of what is involved.  I am still investigating what needs to
be done to fix this.

htdocs/js/ProblemGrader/problemgrader.js

@@ -120,15 +120,17 @@
 					} else {
 						// Update the hidden problem status fields and score table for gateway quizzes
 						if (saveData.versionId !== '0') {
-							document.gwquiz.elements['probstatus' + saveData.problemId].value =
-								parseInt(scoreInput.value) / 100;
+							const probStatus = document.gwquiz.elements[`probstatus${saveData.problemId}`];
+							if (probStatus) probStatus.value = parseInt(scoreInput.value) / 100;
 							let testValue = 0;
 							for (const scoreCell of document.querySelectorAll('table.gwNavigation td.score')) {
 								if (scoreCell.dataset.problemId == saveData.problemId) {
 									scoreCell.textContent = scoreInput.value == '100' ? '\u{1F4AF}' : scoreInput.value;
 								}
-								testValue += document.gwquiz.elements['probstatus'
-									+ scoreCell.dataset.problemId].value * scoreCell.dataset.problemValue;
+								const scoreCellProbStatus =
+									document.gwquiz.elements[`probstatus${scoreCell.dataset.problemId}`];
+								if (scoreCellProbStatus)
+									testValue += scoreCellProbStatus.value * scoreCell.dataset.problemValue;
 							}
 							const recordedScore = document.getElementById('test-recorded-score');
 							if (recordedScore) {