解决通视分析未加载地形时会报错的问题

src/Obj/Analysis/Visibility/index.js

@@ -63,11 +63,15 @@ class VisibilityAnalysis extends Tools {
         that.tip.set_text("左键创建视角终点，右键结束通视分析")
         if (!that.resultObject.viewPoint) {
           let pos84 = that.cartesian3Towgs84(cartesian, that.viewer)
-          let positions = await Cesium.sampleTerrainMostDetailed(
-            that.sdk.viewer.terrainProvider,
-            [Cesium.Cartographic.fromDegrees(pos84.lng, pos84.lat)]
-          );
-          if (positions[0].height > pos84.alt) {
+          let positions
+          if(that.sdk.viewer.terrainProvider.availability)
+          {
+            positions = await Cesium.sampleTerrainMostDetailed(
+              that.sdk.viewer.terrainProvider,
+              [Cesium.Cartographic.fromDegrees(pos84.lng, pos84.lat)]
+            );
+          }
+          if (positions && positions[0].height > pos84.alt) {
             pos84.alt = positions[0].height
           }
           pos84.alt = pos84.alt + that.viewPointHeight