Failed to construct ‘RTCIceCandidate‘ sdpMid and sdpMLineIndex are both null

最近在搞webrtc，在编写函数处理远端传递来的candidate时报错了，具体信息如下。国内关于webrtc的资料很少，所以去国外社区转了一圈，回来记录一下报错的解决方案
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其实这个bug也好解决，根据报错信息可以判断是RTCIceCandidate的sdpMid和sdpMlineIndex为null了，看了看RTCIceCandidate的原码，基本就能判断出应该是对端传递candidate的时候没传递这两个值

interface RTCIceCandidate {
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/address) */
    readonly address: string | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/candidate) */
    readonly candidate: string;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/component) */
    readonly component: RTCIceComponent | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/foundation) */
    readonly foundation: string | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/port) */
    readonly port: number | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/priority) */
    readonly priority: number | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/protocol) */
    readonly protocol: RTCIceProtocol | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/relatedAddress) */
    readonly relatedAddress: string | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/relatedPort) */
    readonly relatedPort: number | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/sdpMLineIndex) */
    readonly sdpMLineIndex: number | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/sdpMid) */
    readonly sdpMid: string | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/tcpType) */
    readonly tcpType: RTCIceTcpCandidateType | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/type) */
    readonly type: RTCIceCandidateType | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/usernameFragment) */
    readonly usernameFragment: string | null;
    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/RTCIceCandidate/toJSON) */
    toJSON(): RTCIceCandidateInit;
}