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This is to fix below issue: when an ongoing transfer is almost to complete,
click the ongoing notification to launch TransferActivity, there will be
possible crash due to NullPointerException.
Root casue is: when TransferActivity onCreate() is called, the transfer is
still ongoing, so mIsComplete is false; but when onCreate() call to
updateProgressbar(), the transfer may change to complete, and then  updateButton()
will be called; In updateButton(),  mAlert (which will be initialized in father
class AlertActivity) still not intialized, which caused NullPointerException.

Change-Id: Ibfe363f3822386a38b9b108e29f60ad6673f70f6