This patch adds a missing language property, service.gui.NO_ONLINE_CONFERENCING_ACCOUNT, to resources.properties.
This property is used when the user selects the Tools > Create a conference call… option without having previously added an account which
supports conferencing. It's usage is specified in src.net.java.sip.communicator.impl.gui.main.menus.ToolsMenu.actionPerformed.
To repro the problem, simply wipe out the current jitsi account data, load Jitsi and click Tools -> Create a conference call… option. The user is then presented with an alert with the text saying 'null'.
The attached patch file was created using git diff > file.patch - if pull requests are easier now that Jitsi has moved to github, I can do that in the future.