FileTransfer Plugin breaks sometimes because result is null

Raw traceback:
TypeError: null is not an object (evaluating 'result.lengthComputable')
at win(/www/plugins/cordova-plugin-file-transfer/www/FileTransfer.js:202:26)
at callbackFromNative(/www/cordova.js:293:63)
at ? (/www/plugins/cordova-plugin-wkwebview-engine/src/www/ios/ios-wkwebview-exec.js:132:32)
at wrapped(raven.js:386:29)

FileTransfer.js in win at line 202:26 result is null. I don't know when or why this happens. But when using `cordova-plugin-wkwebview-engine` and `cordova-labs-wkwebview-engine-localhost` it breaks in about 50% of cases.

Edit:

This error also occurs when not using the plugins mentioned above:

TypeError: null is not an object (evaluating 'result.lengthComputable')
at win(/www/plugins/cordova-plugin-file-transfer/www/FileTransfer.js:202:26)
at callbackFromNative(/www/cordova.js:293:63)
at nc2(/www/cordova.js:1046:39)
at wrapped(raven.js:386:29)