fix: avoid to throw error when link doesn't have parentNode

Author: viezhong

Diff for: src/index.js

@@ -862,7 +862,7 @@ class MiniCssExtractPlugin {
                     'err.code = "CSS_CHUNK_LOAD_FAILED";',
                     "err.type = errorType;",
                     "err.request = realHref;",
-                    "linkTag.parentNode.removeChild(linkTag)",
+                    "if(linkTag.parentNode) linkTag.parentNode.removeChild(linkTag)",
                     "reject(err);",
                   ]),
                   "}",