Fix error in TernUIPlugin shutdown

I'm getting the following error when tern.eclipse.ide.ui bundle shuts down.  This PR contains a simple fix.

```
org.osgi.framework.BundleException: Exception in tern.eclipse.ide.ui.TernUIPlugin.stop() of bundle tern.eclipse.ide.ui.
	at org.eclipse.osgi.internal.framework.BundleContextImpl.stop(BundleContextImpl.java:847)
	at org.eclipse.osgi.internal.framework.EquinoxBundle.stopWorker0(EquinoxBundle.java:950)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$EquinoxModule.stopWorker(EquinoxBundle.java:324)
	at org.eclipse.osgi.container.Module.doStop(Module.java:626)
	at org.eclipse.osgi.container.Module.stop(Module.java:488)
	at org.eclipse.osgi.container.ModuleContainer$ContainerStartLevel.decStartLevel(ModuleContainer.java:1623)
	at org.eclipse.osgi.container.ModuleContainer$ContainerStartLevel.doContainerStartLevel(ModuleContainer.java:1542)
	at org.eclipse.osgi.container.SystemModule.stopWorker(SystemModule.java:248)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$SystemBundle$EquinoxSystemModule.stopWorker(EquinoxBundle.java:145)
	at org.eclipse.osgi.container.Module.doStop(Module.java:626)
	at org.eclipse.osgi.container.Module.stop(Module.java:488)
	at org.eclipse.osgi.container.SystemModule.stop(SystemModule.java:186)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$SystemBundle$EquinoxSystemModule$1.run(EquinoxBundle.java:160)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ExceptionInInitializerError
	at tern.eclipse.ide.ui.TernUIPlugin.getTernDescriptorManager(TernUIPlugin.java:127)
	at tern.eclipse.ide.ui.TernUIPlugin.stop(TernUIPlugin.java:85)
	at org.eclipse.osgi.internal.framework.BundleContextImpl$4.run(BundleContextImpl.java:827)
	at org.eclipse.osgi.internal.framework.BundleContextImpl$4.run(BundleContextImpl.java:1)
	at java.security.AccessController.doPrivileged(Native Method)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.stop(BundleContextImpl.java:820)
	... 13 more
Caused by: java.lang.NullPointerException
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.getTempDir(TernDescriptorManager.java:237)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.<init>(TernDescriptorManager.java:72)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.<clinit>(TernDescriptorManager.java:52)
	... 19 more
Root exception:
java.lang.ExceptionInInitializerError
	at tern.eclipse.ide.ui.TernUIPlugin.getTernDescriptorManager(TernUIPlugin.java:127)
	at tern.eclipse.ide.ui.TernUIPlugin.stop(TernUIPlugin.java:85)
	at org.eclipse.osgi.internal.framework.BundleContextImpl$4.run(BundleContextImpl.java:827)
	at org.eclipse.osgi.internal.framework.BundleContextImpl$4.run(BundleContextImpl.java:1)
	at java.security.AccessController.doPrivileged(Native Method)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.stop(BundleContextImpl.java:820)
	at org.eclipse.osgi.internal.framework.EquinoxBundle.stopWorker0(EquinoxBundle.java:950)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$EquinoxModule.stopWorker(EquinoxBundle.java:324)
	at org.eclipse.osgi.container.Module.doStop(Module.java:626)
	at org.eclipse.osgi.container.Module.stop(Module.java:488)
	at org.eclipse.osgi.container.ModuleContainer$ContainerStartLevel.decStartLevel(ModuleContainer.java:1623)
	at org.eclipse.osgi.container.ModuleContainer$ContainerStartLevel.doContainerStartLevel(ModuleContainer.java:1542)
	at org.eclipse.osgi.container.SystemModule.stopWorker(SystemModule.java:248)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$SystemBundle$EquinoxSystemModule.stopWorker(EquinoxBundle.java:145)
	at org.eclipse.osgi.container.Module.doStop(Module.java:626)
	at org.eclipse.osgi.container.Module.stop(Module.java:488)
	at org.eclipse.osgi.container.SystemModule.stop(SystemModule.java:186)
	at org.eclipse.osgi.internal.framework.EquinoxBundle$SystemBundle$EquinoxSystemModule$1.run(EquinoxBundle.java:160)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.getTempDir(TernDescriptorManager.java:237)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.<init>(TernDescriptorManager.java:72)
	at tern.eclipse.ide.internal.ui.descriptors.TernDescriptorManager.<clinit>(TernDescriptorManager.java:52)
	... 19 more
```