Add missing @Nullable annotations

Summary: Native can return nullptr, so this should be annotated.

Reviewed By: jknoxville

Differential Revision: D15146700

fbshipit-source-id: 332e3a074d2e39cfff677f3d84e94f910563f992

android/src/main/java/com/facebook/flipper/android/FlipperClientImpl.java

@@ -16,6 +16,7 @@ import com.facebook.proguard.annotations.DoNotStrip;
 import com.facebook.soloader.SoLoader;
 import java.util.HashMap;
 import java.util.Map;
+import javax.annotation.Nullable;
 
 @DoNotStrip
 class FlipperClientImpl implements FlipperClient {
@@ -59,9 +60,11 @@ class FlipperClientImpl implements FlipperClient {
    * @deprecated Prefer using {@link #getPluginByClass(Class)} over the stringly-typed interface.
    */
   @Override
+  @Nullable
   @Deprecated
   public native <T extends FlipperPlugin> T getPlugin(String id);
 
+  @Nullable
   @Override
   public <T extends FlipperPlugin> T getPluginByClass(Class<T> cls) {
     final String id = mClassIdentifierMap.get(cls);

android/src/main/java/com/facebook/flipper/core/FlipperClient.java

@@ -7,11 +7,15 @@
  */
 package com.facebook.flipper.core;
 
+import javax.annotation.Nullable;
+
 public interface FlipperClient {
   void addPlugin(FlipperPlugin plugin);
 
+  @Nullable
   <T extends FlipperPlugin> T getPlugin(String id);
 
+  @Nullable
   <T extends FlipperPlugin> T getPluginByClass(Class<T> cls);
 
   void removePlugin(FlipperPlugin plugin);