Downgraded project to 2018.4.

Assets/Scenes/Object Tracking/Cube.arobject.meta

 fileFormatVersion: 2
 guid: 124f01f93d53b4964a2d0b5319c54392
 ScriptedImporter:
-  internalIDToNameTable: []
+  fileIDToRecycleName:
+    11400000: arobject
-  serializedVersion: 2
   userData: 
   assetBundleName: 
   assetBundleVariant: 

Assets/Scenes/Object Tracking/Cylinder.arobject.meta

 fileFormatVersion: 2
 guid: 1c82254aafb5041a290e59ebe31aee5d
 ScriptedImporter:
-  internalIDToNameTable: []
+  fileIDToRecycleName:
+    11400000: arobject
-  serializedVersion: 2
   userData: 
   assetBundleName: 
   assetBundleVariant: 

Packages/manifest.json

 {
   "dependencies": {
-    "com.unity.ads": "3.0.3",
-    "com.unity.analytics": "3.3.2",
-    "com.unity.collab-proxy": "1.2.16",
-    "com.unity.multiplayer-hlapi": "1.0.2",
-    "com.unity.package-manager-ui": "2.1.2",
-    "com.unity.purchasing": "2.0.6",
-    "com.unity.textmeshpro": "2.0.0",
-    "com.unity.timeline": "1.0.0",
-    "com.unity.xr.arfoundation": "2.1.0-preview.2",
+    "com.unity.xr.arfoundation": "1.5.0-preview.2",
-    "com.unity.xr.legacyinputhelpers": "2.0.2",
+    "com.unity.ads": "2.0.8",
+    "com.unity.analytics": "3.2.2",
+    "com.unity.collab-proxy": "1.2.15",
+    "com.unity.package-manager-ui": "2.0.7",
+    "com.unity.purchasing": "2.0.3",
+    "com.unity.textmeshpro": "1.3.0",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.animation": "1.0.0",
     "com.unity.modules.assetbundle": "1.0.0",

ProjectSettings/EditorBuildSettings.asset

   - enabled: 0
     path: Assets/Scenes/Scale.unity
     guid: 13ac91f677dc22f4ca320ef8ae727b68
-  - enabled: 0
+  - enabled: 1
     path: Assets/Scenes/SimpleAR.unity
     guid: 99c9720ab356a0642a771bea13969a05
   - enabled: 0

ProjectSettings/GraphicsSettings.asset

   - {fileID: 10770, guid: 0000000000000000f000000000000000, type: 0}
   - {fileID: 17000, guid: 0000000000000000f000000000000000, type: 0}
   - {fileID: 16000, guid: 0000000000000000f000000000000000, type: 0}
-  - {fileID: 16001, guid: 0000000000000000f000000000000000, type: 0}
+  - {fileID: 16002, guid: 0000000000000000f000000000000000, type: 0}
   m_PreloadedShaders: []
   m_SpritesDefaultMaterial: {fileID: 10754, guid: 0000000000000000f000000000000000,
     type: 0}

ProjectSettings/ProjectVersion.txt

-m_EditorVersion: 2019.1.0f2
-m_EditorVersionWithRevision: 2019.1.0f2 (292b93d75a2c)
+m_EditorVersion: 2018.3.13f1