Guard against null token after trailing backslash in text role

Fixes phpDocumentor/phpDocumentor#4082

Author: lacatoire

packages/guides-restructured-text/src/RestructuredText/Parser/Productions/InlineRules/TextRoleRule.php

@@ -78,6 +78,10 @@ public function apply(BlockContext $blockContext, InlineLexer $lexer): InlineNod
                 case InlineLexer::BACKSLASH:
                     $rawPart .= $token->value;
                     $lexer->moveNext();
+                    if ($lexer->token === null) {
+                        break 2;
+                    }
+
                     $part .= $lexer->token->value;
                     $rawPart .= $lexer->token->value;