Description of problem:
Trying to open a noVNC console for a VM while Firefox is set to block popup windows from the engine url, cause an exception and the console is not opened.


when setting Firefox to allow popups from the engine url, the console is opened as required.
  
Version-Release number of selected component (if applicable):
engine is 4.0
Firefox version is 45.0.1 (but the was reproduced also on Firefow version 24.3.0) 

Steps to Reproduce:
1. Open the "console options" window for a VM and set the "console invocation" to "noVNC"
2. Make sure the Firefox of the web admin/user portal is set to block popup windows from the engine.
3. try to open a console
4. an exception is thrown from GWT and no console is opended.

Additional info:

the exception stack trace:
[ERROR] Thu Apr 14 15:06:03 IDT 2016 remote
[ERROR] SEVERE: Uncaught exception: com.google.gwt.core.client.JavaScriptException: (TypeError) @com.google.gwt.core.client.impl.Impl::apply(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)([JavaScript object(54224), JavaScript object(54223), JavaScript object(54229)]): win is null
[ERROR] 	at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:249)
[ERROR] 	at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
[ERROR] 	at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:576)
[ERROR] 	at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:284)
[ERROR] 	at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
[ERROR] 	at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
[ERROR] 	at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:356)
[ERROR] 	at sun.reflect.GeneratedMethodAccessor53.invoke(Unknown Source)
[ERROR] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR] 	at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR] 	at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
[ERROR] 	at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
[ERROR] 	at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
[ERROR] 	at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:293)
[ERROR] 	at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:547)
[ERROR] 	at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:364)
[ERROR] 	at java.lang.Thread.run(Thread.java:745)