老师为什么这个length报错了

VM2866:1 thirdScriptErrorCannot read property 'length' of undefined;at "pages/movie/movie" page lifeCycleMethod onLoad functionTypeError: Cannot read property 'length' of undefined    at c.onLoad (http://127.0.0.1:37039/appservice/pages/movie/movie.js:20:36)    at c.<anonymous> (http://127.0.0.1:37039/appservice/__dev__/WAService.js:19:16763)    at $t (http://127.0.0.1:37039/appservice/__dev__/WAService.js:19:30369)    at on (http://127.0.0.1:37039/appservice/__dev__/WAService.js:20:235)    at sn (http://127.0.0.1:37039/appservice/__dev__/WAService.js:20:573)    at Function.<anonymous> (http://127.0.0.1:37039/appservice/__dev__/WAService.js:20:3006)    at L.<anonymous> (http://127.0.0.1:37039/appservice/__dev__/WAService.js:19:1500)    at L.emit (http://127.0.0.1:37039/appservice/__dev__/WAService.js:8:15814)    at Object.emit (http://127.0.0.1:37039/appservice/__dev__/WAService.js:6:14059)    at Function.<anonymous> (http://127.0.0.1:37039/appservice/__dev__/WAService.js:9:16174)console.error @ VM2866:1errorReport @ WAService.js:4thirdErrorReport @ WAService.js:4(anonymous) @ WAService.js:4(anonymous) @ WAService.js:19$t @ WAService.js:19on @ WAService.js:20sn @ WAService.js:20(anonymous) @ WAService.js:20(anonymous) @ WAService.js:19L.emit @ WAService.js:8emit @ WAService.js:6(anonymous) @ WAService.js:9(anonymous) @ WAService.js:6e @ appservice?t=1559486584929:4003(anonymous) @ appservice?t=1559486584929:4003(anonymous) @ appservice?t=1559486584929:4003(anonymous) @ appservice?t=1559486584929:4003a.onmessage @ appservice?t=1559486584929:4003