migrate function tries to remove the "tree" property without checking its availability.

Throws an exception in this case:
11:40:46.643 ERROR olia.module.InstallContextImpl: > Could not install or update belgacom-mobile-website module. Task 'Dialog Migration for 5.x' failed. (PathNotFoundException: tree)
info.magnolia.module.delta.TaskExecutionException: Could not Migrate Dialog
at info.magnolia.ui.dialog.setup.DialogMigrationTask.execute(DialogMigrationTask.java:144)
at info.magnolia.module.ModuleManagerImpl.applyDeltas(ModuleManagerImpl.java:512)
at info.magnolia.module.ModuleManagerImpl.installOrUpdateModule(ModuleManagerImpl.java:494)
at info.magnolia.module.ModuleManagerImpl$1.doExec(ModuleManagerImpl.java:277)
at info.magnolia.context.MgnlContext$VoidOp.exec(MgnlContext.java:415)
at info.magnolia.context.MgnlContext$VoidOp.exec(MgnlContext.java:412)
at info.magnolia.context.MgnlContext.doInSystemContext(MgnlContext.java:386)
at info.magnolia.module.ModuleManagerImpl.performInstallOrUpdate(ModuleManagerImpl.java:271)
at info.magnolia.module.ui.ModuleManagerWebUI$2.run(ModuleManagerWebUI.java:127)
at java.lang.Thread.run(Thread.java:680)
Caused by: javax.jcr.PathNotFoundException: tree
at org.apache.jackrabbit.core.NodeImpl$11.perform(NodeImpl.java:2261)
at org.apache.jackrabbit.core.NodeImpl$11.perform(NodeImpl.java:2248)
at org.apache.jackrabbit.core.session.SessionState.perform(SessionState.java:216)
at org.apache.jackrabbit.core.ItemImpl.perform(ItemImpl.java:91)
at org.apache.jackrabbit.core.NodeImpl.getProperty(NodeImpl.java:2248)
at info.magnolia.jcr.wrapper.DelegateNodeWrapper.getProperty(DelegateNodeWrapper.java:242)
at info.magnolia.jcr.decoration.ContentDecoratorNodeWrapper.getProperty(ContentDecoratorNodeWrapper.java:174)
at info.magnolia.jcr.wrapper.DelegateNodeWrapper.getProperty(DelegateNodeWrapper.java:242)
at info.magnolia.jcr.decoration.ContentDecoratorNodeWrapper.getProperty(ContentDecoratorNodeWrapper.java:174)
at info.magnolia.jcr.wrapper.DelegateNodeWrapper.getProperty(DelegateNodeWrapper.java:242)
at info.magnolia.jcr.decoration.ContentDecoratorNodeWrapper.getProperty(ContentDecoratorNodeWrapper.java:174)
at info.magnolia.module.data.setup.migration.DataLinkControlMigrator.migrate(DataLinkControlMigrator.java:93)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.handleField(DialogMigrationTask.java:326)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.handleTab(DialogMigrationTask.java:303)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.handleTabs(DialogMigrationTask.java:260)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.performDialogMigration(DialogMigrationTask.java:216)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.access$000(DialogMigrationTask.java:71)
at info.magnolia.ui.dialog.setup.DialogMigrationTask$1.visit(DialogMigrationTask.java:133)
at info.magnolia.jcr.util.NodeUtil.visit(NodeUtil.java:650)
at info.magnolia.jcr.util.NodeUtil.visit(NodeUtil.java:652)
at info.magnolia.jcr.util.NodeUtil.visit(NodeUtil.java:652)
at info.magnolia.ui.dialog.setup.DialogMigrationTask.execute(DialogMigrationTask.java:129)
... 9 more