Create 2 tables.

Hide the tables.

With the tables hidden, right click and select 'New Relationship'

No exception in ER Modeler, but you can see some logs in ADS , View Log

You can view this in ADS --> Help --> View Log

java.lang.NullPointerException
 at nmfl.aQa.<init>(Unknown Source)
 at nmfl.so.<init>(Unknown Source)
 at nmfl.nz.<init>(Unknown Source)
 at nmfl.Zr.<init>(Unknown Source)
 at nmfl.Zr.a(Unknown Source)
 at nmfl.apE.a(Unknown Source)
 at nmfl.apE.actionPerformed(Unknown Source)
 at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
 at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
 at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
 at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
 at javax.swing.AbstractButton.doClick(AbstractButton.java:357)
 at com.jidesoft.plaf.vsnet.VsnetMenuItemUI.doClick(Unknown Source)
 at com.jidesoft.plaf.vsnet.VsnetMenuItemUI$MouseInputHandler.mouseReleased(Unknown Source)
 at java.awt.Component.processMouseEvent(Component.java:6041)
 at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
 at java.awt.Component.processEvent(Component.java:5806)
 at java.awt.Container.processEvent(Container.java:2058)
 at java.awt.Component.dispatchEventImpl(Component.java:4413)
 at java.awt.Container.dispatchEventImpl(Container.java:2116)
 at java.awt.Component.dispatchEvent(Component.java:4243)
 at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
 at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
 at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
 at java.awt.Container.dispatchEventImpl(Container.java:2102)
 at java.awt.Component.dispatchEvent(Component.java:4243)
 at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
 at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
 at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
 at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
 at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)