Uploading files to Amazon S3 bucket from an Android device throws NullPointerException

Question

I'm attempting to use a Spinner in Android to select a file from a directory, and then upload that file to an Amazon S3 bucket when a button is clicked. However, I'm getting a NullPointerException from the TransferUtility.Upload constructor.

Here's the full error:

AndroidRuntime: FATAL EXCEPTION: main
              Process: [appname], PID: 12322
              java.lang.IllegalStateException: Could not execute method for 
android:onClick
                  at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                  at android.view.View.performClick(View.java:4652)
                  at android.view.View$PerformClick.run(View.java:19455)
                  at android.os.Handler.handleCallback(Handler.java:733)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:146)
                  at android.app.ActivityThread.main(ActivityThread.java:5653)
                  at java.lang.reflect.Method.invokeNative(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:515)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
                  at dalvik.system.NativeStart.main(Native Method)
               Caused by: java.lang.reflect.InvocationTargetException
                  at java.lang.reflect.Method.invokeNative(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:515)
                  at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                  at android.view.View.performClick(View.java:4652) 
                  at android.view.View$PerformClick.run(View.java:19455) 
                  at android.os.Handler.handleCallback(Handler.java:733) 
                  at android.os.Handler.dispatchMessage(Handler.java:95) 
                  at android.os.Looper.loop(Looper.java:146) 
                  at android.app.ActivityThread.main(ActivityThread.java:5653) 
                  at java.lang.reflect.Method.invokeNative(Native Method) 
                  at java.lang.reflect.Method.invoke(Method.java:515) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107) 
                  at dalvik.system.NativeStart.main(Native Method) 
               Caused by: java.lang.NullPointerException
                  at [appname].uploadFileToS3(Analyze.java:193)

And the methods that I'm using:

public void uploadFileToS3(View view){
    // selectedFile is chosen from the spinner's selected position
   File selectedFile = getFile(files);
    //transfer utility
    TransferObserver transferObserver = transferUtility.upload(
            bucket,     /* The bucket to upload to */
            "TEST_FILE.txt",    /* The key for the uploaded object */
            selectedFile     /* The file where the data to upload exists */
    );
    transferObserverListener(transferObserver);
}

private File getFile(ArrayList<File> files) {
    // This is to grab just the file that's located at the spinner's 
    selected position
    int position = fileSpinner.getSelectedItemPosition();
    File selectedfile = files.get(position);
    return selectedfile;
}

"bucket" is a global variable that's set to the name of our S3 bucket. I've passed the file to the TransferUtility constructor manually as well (without using the spinner) and it still threw a NullPointerException. Does anyone have any suggestions?