微信小程序调起外部应用时报：plus is not defined

thirdScriptError
plus is not defined; [Component] Event Handler Error @ pages/map/map#bound handleProxy
ReferenceError: plus is not defined

下面是代码

openMap: function (){  
                let url = ""  
                plus.nativeUI.actionSheet({title: "选择地图应用", cancel: "取消", buttons: [{title:"百度地图"}, {title:"高德地图"}, {title:"腾讯地图"}]}, function(e){   
                    switch (e.index) {  
                        case 1:  
                            url = "baidumap://map/marker?location=39.968789,116.347247&title=DCloud&src=Hello%20uni-app"  
                        break;  
                        case 2:  
                            url = "iosamap://viewMap?sourceApplication=Hello%20uni-app&poiname=DCloud&lat=39.9631018208&lon=116.3406135236&dev=0"  
                        break;  
                    }  
                    if (url!="") {  
                        plus.runtime.openURL( url, function( e ) {   
                            plus.nativeUI.alert("本机未安装指定的地图应用");    
                        });   
                    }  
                })  
            },

<button type="primary" class="cu-btn lg bg-theme" @click="openMap">去这里</button>