fix NPE

[gitseti]

Motivation

ContextSubscribeCommand with -s throws an NPE if the server closes the connection.

E.g:

hmq_joEz7_2_d34e54b40e8e39cddd9929189e78a12f@localhost> sub -t test1 -s
sss
sss
Server closed connection without DISCONNECT.
Press ENTER to resume:
java.lang.NullPointerException
	at com.hivemq.cli.commands.shell.ShellContextCommand.getKey(ShellContextCommand.java:73)
	at com.hivemq.cli.commands.shell.ContextSubscribeCommand.toString(ContextSubscribeCommand.java:166)
	at java.base/java.lang.String.valueOf(String.java:3352)
	at java.base/java.lang.StringBuilder.append(StringBuilder.java:166)
	at picocli.CommandLine.executeUserObject(CommandLine.java:1738)
	at picocli.CommandLine.access$900(CommandLine.java:145)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2101)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2068)
	at picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:1935)
	at picocli.CommandLine.execute(CommandLine.java:1864)
	at com.hivemq.cli.commands.shell.ShellCommand.interact(ShellCommand.java:153)
	at com.hivemq.cli.commands.shell.ShellCommand.run(ShellCommand.java:103)
	at picocli.CommandLine.executeUserObject(CommandLine.java:1729)
	at picocli.CommandLine.access$900(CommandLine.java:145)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2101)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2068)
	at picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:1935)
	at picocli.CommandLine.execute(CommandLine.java:1864)
	at com.hivemq.cli.MqttCLIMain.main(MqttCLIMain.java:59)
mqtt>
mqtt>

Changes

• Add not null checks