FFMPEG write header error

See more:

C++

ffmpeg

Hi, I'm working on a FFMPEg Project and trying to write a file with a stream. But when I call avformat_write_header it always throws an errr. Here's the code:

AVCodecContext *codecctx;
	AVFormatContext *fmtctx;
	fmt=av_guess_format(NULL,filename,NULL);
	if(!fmt)
		printf("Das Format kann nicht erkannt werden. MPEG wird verwendet");
	AVCodec *codec = avcodec_find_encoder(fmt->video_codec);
	codecctx = avcodec_alloc_context3(codec);
	codecctx->codec_id = fmt->video_codec;
	codecctx->codec_type = AVMEDIA_TYPE_VIDEO;
	codecctx->gop_size = 12;
	codecctx->bit_rate = WIDTH*HEIGHT*4;
	codecctx->width = WIDTH;
	codecctx->height = HEIGHT;
	codecctx->time_base.den = 2; // Framerate kann man nachträglich verändern. 0.2 hier bedeutet dass jedes Bild für 5 Sekunden erscheint. 
	codecctx->time_base.num =1;
	//codecctx->time_base =(AVRational){1,FPS};
	codecctx->max_b_frames=1;
	codecctx->pix_fmt= STREAM_PIX_FMT;
	fmtctx = avformat_alloc_context();
    fmtctx->oformat = fmt;
    fmtctx->video_codec_id = fmt->video_codec;
	avcodec_open2(codecctx, codec, NULL);
	if(avcodec_open2(codecctx, codec, NULL)<0)
		printf("Codec nicht geoeffnet");
	AVStream *VideoStream = avformat_new_stream(fmtctx,0);
	VideoStream->codecpar->codec_id=fmt->video_codec;
	VideoStream->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
	VideoStream->codecpar->format = STREAM_PIX_FMT;
	VideoStream->codecpar->width=WIDTH;
	VideoStream->codecpar->height=HEIGHT;
	VideoStream->time_base.den=0.2;
	VideoStream->time_base.num=1;
	
	avformat_write_header(fmtctx,0);

And here's the error:
Unhandled exception at 0x0070848D (avformat-57.dll) in ConsoleApplication1.exe: 0xC0000005: Access violation reading location 0x0000000C.

What I have tried:

I've try to remove the write Header line and it runs well.