Crash when sending an alert while the initial screen is displayed.

[Andreas Preikschat]

Hello,

trunk r1788

1) Start OpenLP
2) Hide the screen
3) Press F7 to display an alert message

Result:
Traceback (most recent call last):
  File "/home/andreas/Projekte/openlp/trunk/openlp/plugins/alerts/forms/alertform.py", line 84, in onDisplayClicked
    self.triggerAlert(unicode(self.alertTextEdit.text()))
  File "/home/andreas/Projekte/openlp/trunk/openlp/plugins/alerts/forms/alertform.py", line 198, in triggerAlert
    self.plugin.alertsmanager.displayAlert(text)
  File "/home/andreas/Projekte/openlp/trunk/openlp/plugins/alerts/lib/alertsmanager.py", line 77, in displayAlert
    self.generateAlert()
  File "/home/andreas/Projekte/openlp/trunk/openlp/plugins/alerts/lib/alertsmanager.py", line 88, in generateAlert
    self.parent().liveController.display.alert(text, alertTab.location)
  File "/home/andreas/Projekte/openlp/trunk/openlp/core/ui/maindisplay.py", line 242, in alert
    alert_height = int(height.toString())
ValueError: invalid literal for int() with base 10: ''

The reason is that at the time of building the html for the initial screen the plugins are not loaded OR the css/html is just not considered.