Fix NPE when platform plugin delegate is null (flutter/engine#49391)

Fixes: 
https://github.com/flutter/flutter/issues/140600
https://github.com/flutter/flutter/issues/140031

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engine/src/flutter/shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -38,7 +38,7 @@ public class PlatformPlugin {
 
   private final Activity activity;
   private final PlatformChannel platformChannel;
-  private final PlatformPluginDelegate platformPluginDelegate;
+  @Nullable private final PlatformPluginDelegate platformPluginDelegate;
   private PlatformChannel.SystemChromeStyle currentTheme;
   private int mEnabledOverlays;
   private static final String TAG = "PlatformPlugin";
@@ -161,7 +161,7 @@ public class PlatformPlugin {
   public PlatformPlugin(
       @NonNull Activity activity,
       @NonNull PlatformChannel platformChannel,
-      @NonNull PlatformPluginDelegate delegate) {
+      @Nullable PlatformPluginDelegate delegate) {
     this.activity = activity;
     this.platformChannel = platformChannel;
     this.platformChannel.setPlatformMessageHandler(mPlatformMessageHandler);
@@ -499,7 +499,9 @@ public class PlatformPlugin {
   }
 
   private void setFrameworkHandlesBack(boolean frameworkHandlesBack) {
-    platformPluginDelegate.setFrameworkHandlesBack(frameworkHandlesBack);
+    if (platformPluginDelegate != null) {
+      platformPluginDelegate.setFrameworkHandlesBack(frameworkHandlesBack);
+    }
   }
 
   private void popSystemNavigator() {

engine/src/flutter/shell/platform/android/test/io/flutter/plugin/platform/PlatformPluginTest.java

@@ -11,6 +11,7 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 import static org.mockito.Mockito.any;
 import static org.mockito.Mockito.anyBoolean;
 import static org.mockito.Mockito.doThrow;
@@ -613,6 +614,20 @@ public class PlatformPluginTest {
     verify(mockPlatformPluginDelegate, times(1)).setFrameworkHandlesBack(true);
   }
 
+  @Test
+  public void testPlatformPluginDelegateNull() throws Exception {
+    Activity mockActivity = mock(Activity.class);
+    PlatformPlugin platformPlugin =
+        new PlatformPlugin(mockActivity, mockPlatformChannel, null /*platformPluginDelegate*/);
+
+    try {
+      platformPlugin.mPlatformMessageHandler.setFrameworkHandlesBack(true);
+    } catch (NullPointerException e) {
+      // Not expected
+      fail("NullPointerException was thrown");
+    }
+  }
+
   @Test
   public void popSystemNavigatorFlutterActivity() {
     Activity mockActivity = mock(Activity.class);