Fix missing null check in collectRootScrollCaptureTargets

If no window-level handlers have been added, the list is null
(lazily init).

Test: atest com.android.systemui.screenshot.ScrollCaptureTest

Change-Id: I39d4295cebb091fa89f65c5917ba08bc2396ed37
diff --git a/packages/SystemUI/tests/AndroidManifest.xml b/packages/SystemUI/tests/AndroidManifest.xml
index c6331682..db87845 100644
--- a/packages/SystemUI/tests/AndroidManifest.xml
+++ b/packages/SystemUI/tests/AndroidManifest.xml
@@ -80,6 +80,8 @@
                   android:excludeFromRecents="true"
                   />
 
+        <activity android:name="com.android.systemui.screenshot.ScrollViewActivity"
+                  android:exported="false" />
         <provider
             android:name="androidx.lifecycle.ProcessLifecycleOwnerInitializer"
             tools:replace="android:authorities"