Fixing potential issue that fails to create AssetReference when asset type of asset on given path is null.

Editor/System/Core/AssetGraphPostprocessor.cs

@@ -105,8 +105,9 @@ namespace UnityEngine.AssetGraph {
                 }
             }            
 
-            foreach (string str in importedAssets) {
-                AssetReferenceDatabase.GetReference (str).InvalidateTypeCache();
+            foreach (string str in importedAssets)
+            {
+                AssetReferenceDatabase.GetReference(str)?.InvalidateTypeCache();
             }
 
             foreach (string str in deletedAssets) 

Editor/System/Model/AssetReferenceDatabase.cs

@@ -133,7 +133,9 @@ namespace UnityEngine.AssetGraph {
 					db.m_dictionary.Add(relativePath, r);
 					AssetReferenceDatabase.SetDBDirty();
 					return r;
-				} catch(AssetReferenceException ) {
+				} catch(AssetReferenceException e) {
+					LogUtility.Logger.LogWarning (LogUtility.kTag,
+						$"AssetReference could not be created: Asset:{e.importFrom} Message:{e.Message}");
 					// if give asset is invalid, return null
 					return null;
 				}