Fix unneeded QObject::connect which was failing

The connect call was generating debug output to identify that it was
failing. After looking at the code realized that this connect is not
needed. The group box is removed when UASListWidget::removeUAS method
is called.

Second change in the file fixes a crashing bug when the last UAS is
removed. Need to exit the iterator after we modify it.
Fix crash after last UAS is removed

ui->waitingLabel is deleted and set to NULL when mode is PX4. Must
check for NULL before using it.
Merge pull request #564 from DonLakeFlyer/ConnectFix

Failed QObject::Connect fix