error alert persist

[xgui3783]

as of c0b49f6

steps to reproduce:

• select a volume, go to edit landmarks > load landmarks
• attempt to load a landmark.json that is invalid, you should see an error alert that json is invalid
• load a landmark.json that is valid
• close load landmarks
• reopen load landmarks

expected result:

• error alert does not exist

actual result:

• an empty error alert exists