Handle SampleHeightMostDetailed on tilesets that completely fail to load, and add a test.

Source/CesiumRuntime/Private/Cesium3DTileset.cpp

@@ -142,19 +142,30 @@ void ACesium3DTileset::SampleHeightMostDetailed(
         position.Z));
   }
 
+  size_t count = positions.size();
+
   CesiumAsync::Future<Cesium3DTilesSelection::SampleHeightResult> future =
       this->_pTileset
           ? this->_pTileset->sampleHeightMostDetailed(positions)
           : getAsyncSystem().createResolvedFuture(
                 Cesium3DTilesSelection::SampleHeightResult{
                     std::move(positions),
-                    std::vector<bool>(positions.size(), false),
+                    std::vector<bool>(count, false),
                     {"Could not sample heights from tileset because it has not "
                      "been created."}});
 
-  std::move(future).thenImmediately(
-      [this, OnHeightsSampled = std::move(OnHeightsSampled)](
-          Cesium3DTilesSelection::SampleHeightResult&& result) {
+  std::move(future)
+      .catchImmediately([positions = std::move(positions)](
+                            std::exception&& exception) mutable {
+        size_t count = positions.size();
+        return Cesium3DTilesSelection::SampleHeightResult{
+            std::move(positions),
+            std::vector<bool>(count, false),
+            {exception.what()}};
+      })
+      .thenImmediately([this, OnHeightsSampled = std::move(OnHeightsSampled)](
+                           Cesium3DTilesSelection::SampleHeightResult&&
+                               result) {
         if (!IsValid(this))
           return;
 

Source/CesiumRuntime/Private/Tests/SampleHeightMostDetailed.spec.cpp

@@ -298,4 +298,59 @@ void FSampleHeightMostDetailedSpec::Define() {
                   }));
         });
   });
+
+  Describe("Broken tileset", [this]() {
+    BeforeEach([this]() { CesiumTestHelpers::pushAllowTickInEditor(); });
+
+    AfterEach(EAsyncExecution::TaskGraphMainThread, [this]() {
+      CesiumTestHelpers::popAllowTickInEditor();
+    });
+
+    LatentIt(
+        "",
+        EAsyncExecution::TaskGraphMainThread,
+        [this](const FDoneDelegate& done) {
+          // Two slightly different error messages will occur, depending on
+          // whether there's a web server running on localhost.
+          this->AddExpectedError(
+              TEXT("(Errors when loading)|(error occurred)"));
+
+          UWorld* pWorld = CesiumTestHelpers::getGlobalWorldContext();
+
+          ACesium3DTileset* pTileset = pWorld->SpawnActor<ACesium3DTileset>();
+          pTileset->SetTilesetSource(ETilesetSource::FromUrl);
+          pTileset->SetUrl("http://localhost/notgonnawork");
+
+          pTileset->SampleHeightMostDetailed(
+              {FVector(-105.1, 40.1, 1.0)},
+              FCesiumSampleHeightMostDetailedCallback::CreateLambda(
+                  [this, done](
+                      ACesium3DTileset* pTileset,
+                      const TArray<FCesiumSampleHeightResult>& result,
+                      const TArray<FString>& warnings) {
+                    TestEqual("Number of results", result.Num(), 1);
+                    TestEqual("Number of warnings", warnings.Num(), 1);
+                    TestFalse("SampleSuccess", result[0].SampleSuccess);
+                    TestEqual(
+                        "Longitude",
+                        result[0].LongitudeLatitudeHeight.X,
+                        -105.1,
+                        1e-12);
+                    TestEqual(
+                        "Latitude",
+                        result[0].LongitudeLatitudeHeight.Y,
+                        40.1,
+                        1e-12);
+                    TestEqual(
+                        "Height",
+                        result[0].LongitudeLatitudeHeight.Z,
+                        1.0,
+                        1e-12);
+                    TestTrue(
+                        "Error message",
+                        warnings[0].Contains(TEXT("failed to load")));
+                    done.ExecuteIfBound();
+                  }));
+        });
+  });
 }