Avoid un-registration when mRealInstrumentation is null.

bug: 259957834
Test: http://ab/I62800010122127541
Change-Id: Ibf43986b2aba6542c20551e9414f1471ec81215a

packages/SystemUI/tests/utils/src/com/android/systemui/SysuiTestCase.java

@@ -132,8 +132,10 @@ public abstract class SysuiTestCase {
 
     @After
     public void SysuiTeardown() {
-        InstrumentationRegistry.registerInstance(mRealInstrumentation,
-                InstrumentationRegistry.getArguments());
+        if (mRealInstrumentation != null) {
+            InstrumentationRegistry.registerInstance(mRealInstrumentation,
+                    InstrumentationRegistry.getArguments());
+        }
         if (TestableLooper.get(this) != null) {
             TestableLooper.get(this).processAllMessages();
             // Must remove static reference to this test object to prevent leak (b/261039202)