added check null fragment on RxActivityResult

Author: Victor

README.md

@@ -23,7 +23,7 @@ allprojects {
 And add next dependencies in the build.gradle of the module:
 ```gradle
 dependencies {
-    compile "com.github.VictorAlbertos:RxActivityResult:0.3.1"
+    compile "com.github.VictorAlbertos:RxActivityResult:0.3.2"
     compile "io.reactivex:rxjava:1.1.0"
 }
 ```

rx_activity_result/src/main/java/rx_activity_result/RxActivityResult.java

@@ -142,7 +142,7 @@ private OnResult onResultFragment() {
             for (Fragment fragment : fragments) {
                 if(fragment != null && fragment.isVisible() && fragment.getClass() == clazz) {
                     return fragment;
-                } else if (fragment.getChildFragmentManager() != null) {
+                } else if (fragment != null && fragment.getChildFragmentManager() != null) {
                     List<Fragment> childFragments = fragment.getChildFragmentManager().getFragments();
                     return getTargetFragment(childFragments);
                 }