NPE in ImageResource.registerImageDescriptor()

Image Registry is not initialized on a first use, the following exception is thrown as result:

 java.lang.NullPointerException
	at tern.eclipse.ide.ui.ImageResource.registerImageDescriptor(ImageResource.java:161)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptor.getImageKey(TernDescriptor.java:38)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptor.<init>(TernDescriptor.java:21)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.addTernDescriptors(TernDescriptorManager.java:116)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.loadTernDescriptors(TernDescriptorManager.java:101)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.getTernDescriptor(TernDescriptorManager.java:82)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.getImage(TernDescriptorManager.java:72)
	at tern.eclipse.ide.ui.contentassist.JSTernCompletionProposal.getDefaultImage(JSTernCompletionProposal.java:61)
	at tern.eclipse.jface.contentassist.TernCompletionProposal.<init>(TernCompletionProposal.java:56)
	at tern.eclipse.ide.ui.contentassist.JSTernCompletionProposal.<init>(JSTernCompletionProposal.java:50)
	at tern.eclipse.ide.ui.contentassist.JSTernCompletionCollector.addProposal(JSTernCompletionCollector.java:38)
	at tern.server.AbstractTernServer.addProposal(AbstractTernServer.java:103)
	at tern.server.nodejs.NodejsTernServer.request(NodejsTernServer.java:246)
	at tern.eclipse.ide.core.IDETernProject.request(IDETernProject.java:511)
	at tern.eclipse.ide.core.IDETernProject.request(IDETernProject.java:505)
	at tern.eclipse.ide.jsdt.internal.contentassist.TernContentAssistProcessor.computeCompletionProposals(TernContentAssistProcessor.java:78)
	at org.eclipse.wst.sse.ui.internal.contentassist.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:284)
	at org.eclipse.wst.sse.ui.internal.contentassist.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:290)
	at org.eclipse.wst.sse.ui.contentassist.StructuredContentAssistProcessor.collectProposals(StructuredContentAssistProcessor.java:484)
	at org.eclipse.wst.sse.ui.contentassist.StructuredContentAssistProcessor.computeCompletionProposals(StructuredContentAssistProcessor.java:255)
	at org.eclipse.wst.sse.ui.internal.contentassist.CompoundContentAssistProcessor.computeCompletionProposals(CompoundContentAssistProcessor.java:127)
	at org.eclipse.jface.text.contentassist.ContentAssistant.computeCompletionProposals(ContentAssistant.java:1861)
	at org.eclipse.jface.text.contentassist.CompletionProposalPopup.computeProposals(CompletionProposalPopup.java:573)
	at org.eclipse.jface.text.contentassist.CompletionProposalPopup.access$16(CompletionProposalPopup.java:570)
	at org.eclipse.jface.text.contentassist.CompletionProposalPopup$2.run(CompletionProposalPopup.java:505)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
	at org.eclipse.jface.text.contentassist.CompletionProposalPopup.showProposals(CompletionProposalPopup.java:499)
	at org.eclipse.jface.text.contentassist.ContentAssistant$2.run(ContentAssistant.java:378)
	at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
	at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:136)
	at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:3781)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3419)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$9.run(PartRenderingEngine.java:1152)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1033)
	at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:148)
	at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:635)
	at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
	at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:578)
	at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:150)
	at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:135)
	at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
	at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:379)
	at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:233)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:648)
	at org.eclipse.equinox.launcher.Main.basicRun(Main.java:603)
	at org.eclipse.equinox.launcher.Main.run(Main.java:1465)
	at org.eclipse.equinox.launcher.Main.main(Main.java:1438)


 An additional check and the initialization (if needed) were added in order to fix the issue.

Signed-off-by: vrubezhny <vrubezhny@exadel.com>