fix(app-plus): 修复subNVues不使用type: popup，使用setStyle设置mask报错

src/platforms/app-plus/service/api/plugin/sub-nvue.js

@@ -62,9 +62,9 @@ function wrapper (webview) {
 }
 
 export function getSubNVueById (id) {
-  const webview = plus.webview.getWebviewById(id)
-  if (webview === null || webview === undefined) {
-    throw new Error('Unable to find SubNVue, id=' + id)
+  const webview = plus.webview.getWebviewById(id)
+  if (webview === null || webview === undefined) {
+    throw new Error('Unable to find SubNVue, id=' + id)
   }
   if (webview && !webview.$processed) {
     wrapper(webview)
@@ -73,7 +73,7 @@ export function getSubNVueById (id) {
   var parentWebview = plus.webview.getWebviewById(webview.__uniapp_mask_id)
   webview.setStyle = function (style) {
     if (style && style.mask) {
-      parentWebview.setStyle({
+      parentWebview && parentWebview.setStyle({
         mask: style.mask
       })
       delete style.mask
@@ -85,4 +85,4 @@ export function getSubNVueById (id) {
 
 export function getCurrentSubNVue () {
   return getSubNVueById(plus.webview.currentWebview().id)
-}
+}