Worked around null PluginImporter being returned by some assets.

In Unity 2018.4.13 AssetImporter.GetAtPath() is returning null for some
assets in the project. This works around the issue by not inspecting
importer state for assets without a PluginImporter.

Change-Id: I4e8e2d0f3ce04997f716305ce1d9ebed36e8739f

Author: Stewart Miles

source/VersionHandlerImpl/src/VersionHandlerImpl.cs

@@ -1013,6 +1013,9 @@ public bool EnableMostRecentPlugins(HashSet<string> disableFiles) {
                 } catch (InvalidCastException) {
                     continue;
                 }
+                if (pluginImporter == null) {
+                    continue;
+                }
                 bool editorEnabled = metadata.GetEditorEnabled();
                 var selectedTargets = metadata.GetBuildTargets();
                 var hasBuildTargets = metadata.GetBuildTargetsSpecified();