[tradfri] NPE if gateway is unreachable

[t2000]

In case the IKEA gateway is unreachable the binding runs into an NPE:

2021-01-01 15:31:32.243 [ERROR] [nal.common.AbstractInvocationHandler] - An error occurred while calling method 'ThingHandler.initialize()' on 'org.openhab.binding.tradfri.internal.handler.TradfriControllerHandler@6f16b0c4': null
java.lang.NullPointerException: null
        at org.eclipse.californium.core.CoapClient.setEndpoint(CoapClient.java:354) ~[?:?]
        at org.openhab.binding.tradfri.internal.handler.TradfriThingHandler.initialize(TradfriThingHandler.java:73) ~[?:?]
        at jdk.internal.reflect.GeneratedMethodAccessor58.invoke(Unknown Source) ~[?:?]
        at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
        at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
        at org.openhab.core.internal.common.AbstractInvocationHandler.invokeDirect(AbstractInvocationHandler.java:154) [bundleFile:?]
        at org.openhab.core.internal.common.Invocation.call(Invocation.java:52) [bundleFile:?]
        at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
        at java.lang.Thread.run(Thread.java:834) [?:?]

The reason being that handler.getEndpoint() returns null here:

openhab-addons/bundles/org.openhab.binding.tradfri/src/main/java/org/openhab/binding/tradfri/internal/handler/TradfriThingHandler.java

Lines 67 to 73 in d3ae641

	TradfriGatewayHandler handler = (TradfriGatewayHandler) tradfriGateway.getHandler();
	String uriString = handler.getGatewayURI() + "/" + id;
	try {
	URI uri = new URI(uriString);
	coapClient = new TradfriCoapClient(uri);
	coapClient.setEndpoint(handler.getEndpoint());

I noticed that the gateway handler (bridge) does not have an endpoint if it is OFFLINE.

All happened during the initialization, so after the bridge goes OFFLINE the initialization of the handlers is triggered and then runs into the NPE.

2021-01-01 15:31:32.228 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'tradfri:gateway:ikeagw' changed from INITIALIZING to OFFLINE (COMMUNICATION_ERROR): No response from gateway. Might be due to an invalid security code.
2021-01-01 15:31:32.233 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'tradfri:0830:ikeagw:ikeaRemote' changed from UNINITIALIZED (BRIDGE_UNINITIALIZED) to INITIALIZING
2021-01-01 15:31:32.235 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'tradfri:0210:ikeagw:ikeaColor' changed from UNINITIALIZED (BRIDGE_UNINITIALIZED) to INITIALIZING
2021-01-01 15:31:32.261 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'tradfri:0830:ikeagw:ikeaRemote' changed from INITIALIZING to UNINITIALIZED (HANDLER_INITIALIZING_ERROR)
2021-01-01 15:31:32.261 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'tradfri:0210:ikeagw:ikeaColor' changed from INITIALIZING to UNINITIALIZED (HANDLER_INITIALIZING_ERROR)

I will provide a PR that sets the coapEndpoint only if the bridge is ONLINE.