Prevent NPE on ThemedReactContext

Summary:
When pre-prerendering a surface without rendering it, "SurfaceMountingManager.getContext()" will return null since no ThemedReactContext is passed into it:
https://www.internalfb.com/code/fbsource/[0f4812c8039d975b3651dfa756d60a882bf8e193]/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java?lines=298-317

Changelog:
[Android][Fixed] - Prevent NPE on ThemedReactContext

Reviewed By: ShikaSD

Differential Revision: D31691022

fbshipit-source-id: 4cca6c4550a77075f8fd754dbf21fe5bf5da77e6

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -559,6 +559,11 @@ public boolean getThemeData(int surfaceId, float[] defaultTextInputPadding) {
     SurfaceMountingManager surfaceMountingManager =
         mMountingManager.getSurfaceManagerEnforced(surfaceId, "getThemeData");
     ThemedReactContext themedReactContext = surfaceMountingManager.getContext();
+
+    if (themedReactContext == null) {
+      FLog.w(TAG, "\"themedReactContext\" is null when call \"getThemeData\"");
+      return false;
+    }
     float[] defaultTextInputPaddingForTheme =
         UIManagerHelper.getDefaultTextInputPadding(themedReactContext);
     defaultTextInputPadding[0] = defaultTextInputPaddingForTheme[PADDING_START_INDEX];