Android FragmentManager BackStackRecord.run throwing NullPointerException
android, android-fragments, fragmentmanager, java
Solution
Answering my own question:
This exception is (eventually) thrown when you call `FragmentTransaction.remove(null);` and `FragmentTransaction.commit();`
EDIT: And also, like Twice Circled and shinyuX point out in the comment; when calling the `show(null)` or `add(null)`, `attach(null)` and `detach(null)` methods, and probably also `hide(null)`
After calling `commit()`, the transaction will be queued in the FragmentManager. As a result, when the operation is being processed after you explicitly call `FragmentManager.executePendingTransactions()`, or when the FragmentManager queue thread calls it, it throws a `NullPointerException`.
In my case, I was maintaining fragment states in a global object. There I checked if the fragment was showing or not, and then removed visible fragments. But because I started a new FragmentActivity, these states were still set to true while they were not visible. So this is a design error.
Other than fixing the design error, the solution was simple: check whether `FragmentManager.findFragmentByTag()` returned `null` before removing the fragment.
Problem
I sometimes get the following exception when working with Fragments: ``` FATAL EXCEPTION: main java.lang.NullPointerException at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:591) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1416) at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:420) at android.os.Handler.handleCallback(Handler.java:615) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4745) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) at dalvik.system.NativeStart.main(Native Method) ``` The exception occurs when `run()` of `BackStackRecord` is called through `execPendingTransactions()`, when it tries to remove a fragment from the manager. ``` case OP_REMOVE: { Fragment f = op.fragment; f.mNextAnim = op.exitAnim; <---- mManager.removeFragment(f, mTransition, mTransitionStyle); } break; ``` I can't seem to figure out what exactly is causing this? I think it has to do with the backstack of fragments not being cleaned up right when removing fragments.