Don't break when no content-type is set on a response

operations/development-server.js

@@ -18,7 +18,9 @@ function monkeyPatchEnd(res) {
 		let prefix;
 
 		if (originalChunk == null || typeof originalChunk === "string" || Buffer.isBuffer(originalChunk)) {
-			if (res.getHeader("content-type").startsWith("text/html")) {
+			let typeHeader = res.getHeader("content-type");
+
+			if (typeHeader != null && typeHeader.startsWith("text/html")) {
 				let contentLength = res.getHeader("content-length");
 				
 				if (contentLength != null) {