Fix potential nullable accesss

Summary:
While ramping up to Flipper I found this potential NPE caused by ordering issues.

There are only 3 uses downstream that don't use a constant that's part of the default initializer.
```
λ festevezga-mbp xplat → buck build sonar/android/src/main/java/com/facebook/flipper/plugins/inspector/... 2>&1 | rg ' = descriptorForClass' | awk '{$1=$1};1' | sort | uniq -c
   5 NodeDescriptor descriptor = descriptorForClass(Object.class);
   3 NodeDescriptor descriptor = descriptorForClass(View.class);
   1 NodeDescriptor descriptor = descriptorForClass(node.getClass());
   2 final NodeDescriptor descriptor = descriptorForClass(Dialog.class);
  12 final NodeDescriptor descriptor = descriptorForClass(Fragment.class);
   5 final NodeDescriptor descriptor = descriptorForClass(Object.class);
  26 final NodeDescriptor descriptor = descriptorForClass(View.class);
   2 final NodeDescriptor descriptor = descriptorForClass(Window.class);
   1 final NodeDescriptor descriptor = descriptorForClass(node.getClass());
   1 final NodeDescriptor descriptor = descriptorForClass(topChild.getClass());
   2 final NodeDescriptor objectDescriptor = descriptorForClass(Object.class);
```

Reviewed By: muraziz

Differential Revision: D22284669

fbshipit-source-id: b41c8d78c70c5b5acdc08aa6f0e7afa681f4242d

android/src/main/java/com/facebook/flipper/plugins/inspector/DescriptorMapping.java

@@ -14,6 +14,7 @@ import android.view.View;
 import android.view.ViewGroup;
 import android.view.Window;
 import android.widget.TextView;
+import androidx.annotation.Nullable;
 import com.facebook.flipper.core.FlipperConnection;
 import com.facebook.flipper.plugins.inspector.descriptors.ActivityDescriptor;
 import com.facebook.flipper.plugins.inspector.descriptors.ApplicationDescriptor;
@@ -66,7 +67,7 @@ public class DescriptorMapping {
     mMapping.put(clazz, descriptor);
   }
 
-  public NodeDescriptor<?> descriptorForClass(Class<?> clazz) {
+  public @Nullable NodeDescriptor<?> descriptorForClass(Class<?> clazz) {
     while (!mMapping.containsKey(clazz)) {
       clazz = clazz.getSuperclass();
     }

android/src/main/java/com/facebook/flipper/plugins/inspector/NodeDescriptor.java

@@ -17,13 +17,15 @@ import java.util.Collections;
 import java.util.List;
 
 /**
- * A NodeDescriptor is an object which known how to expose an Object of type T to the ew Inspector.
+ * A NodeDescriptor is an object which known how to expose an Object of type T to the new Inspector.
  * This class is the extension point for the Flipper inspector plugin and is how custom classes and
  * data can be exposed to the inspector.
  */
 public abstract class NodeDescriptor<T> {
   @Nullable protected FlipperConnection mConnection;
-  private DescriptorMapping mDescriptorMapping;
+
+  // This field is not initialized until setDescriptorMapping is called
+  @Nullable private DescriptorMapping mDescriptorMapping;
 
   void setConnection(FlipperConnection connection) {
     mConnection = connection;
@@ -39,9 +41,12 @@ public abstract class NodeDescriptor<T> {
    *     object it describes. This is highly encouraged instead of subclassing another descriptor
    *     class.
    */
-  protected final NodeDescriptor<?> descriptorForClass(Class<?> clazz) {
+  protected final @Nullable NodeDescriptor<?> descriptorForClass(Class<?> clazz) {
+    if (mDescriptorMapping != null) {
       return mDescriptorMapping.descriptorForClass(clazz);
     }
+    return null;
+  }
 
   /**
    * Invalidate a node. This tells Flipper that this node is no longer valid and its properties

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/ApplicationDescriptor.java

@@ -230,9 +230,11 @@ public class ApplicationDescriptor extends NodeDescriptor<ApplicationWrapper> {
     if (childCount > 0) {
       final Object topChild = getChildAt(node, childCount - 1);
       final NodeDescriptor descriptor = descriptorForClass(topChild.getClass());
+      if (descriptor != null) {
         descriptor.setHighlighted(topChild, selected, isAlignmentMode);
       }
     }
+  }
 
   private void runHitTest(ApplicationWrapper node, Touch touch, boolean ax) {
     final int childCount = getChildCount(node);

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/ObjectDescriptor.java

@@ -70,7 +70,8 @@ public class ObjectDescriptor extends NodeDescriptor<Object> {
   @Override
   public boolean matches(String query, Object node) throws Exception {
     final NodeDescriptor descriptor = descriptorForClass(node.getClass());
-    final List<Named<String>> attributes = descriptor.getAttributes(node);
+    final List<Named<String>> attributes =
+        descriptor == null ? Collections.emptyList() : descriptor.getAttributes(node);
     for (Named<String> namedString : attributes) {
       if (namedString.getName().equals("id")) {
         if (namedString.getValue().toLowerCase().contains(query)) {
@@ -79,6 +80,6 @@ public class ObjectDescriptor extends NodeDescriptor<Object> {
       }
     }
 
-    return descriptor.getName(node).toLowerCase().contains(query);
+    return descriptor == null ? false : descriptor.getName(node).toLowerCase().contains(query);
   }
 }

android/src/main/java/com/facebook/flipper/plugins/inspector/descriptors/ViewGroupDescriptor.java

@@ -74,7 +74,9 @@ public class ViewGroupDescriptor extends NodeDescriptor<ViewGroup> {
               if (connected()) {
                 if (key.set(node)) {
                   NodeDescriptor descriptor = descriptorForClass(node.getClass());
+                  if (descriptor != null) {
                     descriptor.invalidate(node);
+                  }
                   invalidateAX(node);
                 }