Check that terminal part exists when accessing it (#1594)

It was previously trying to access properties on undefined when
getTerminalParts would return an empty array due to an error parsing
the RegExp (because the parser doesn't currently support lookbehind).
The error only occurred with hidden terminals

packages/langium-cli/src/generator/highlighting/monarch-generator.ts

@@ -328,7 +328,7 @@ function getWhitespaceRules(grammar: Grammar): Rule[] {
             const part = RegExpUtils.getTerminalParts(regex)[0];
 
             // check if this is a comment terminal w/ a start & end sequence (multi-line)
-            if (part.start !== '' && part.end !== '' && GrammarUtils.isCommentTerminal(rule)) {
+            if (part && part.start !== '' && part.end !== '' && GrammarUtils.isCommentTerminal(rule)) {
                 // state-based comment rule, only add push to jump into it
                 rules.push({
                     regex: part.start,
@@ -359,7 +359,7 @@ function getCommentRules(grammar: Grammar): Rule[] {
         if (GrammarAST.isTerminalRule(rule) && GrammarUtils.isCommentTerminal(rule)) {
             const tokenName = 'comment';
             const part = RegExpUtils.getTerminalParts(GrammarUtils.terminalRegex(rule))[0];
-            if (part.start !== '' && part.end !== '') {
+            if (part && part.start !== '' && part.end !== '') {
                 // rules to manage comment start/end
                 // rule order matters