Prevent NPE on Missing/Invalid Java Platform

[lkishalmi]

In the hopefully rare case, when a JDK got removed from the system between Gradle project reloads, platform can go invalid and an attempted registration throws an NPE at:

java.lang.NullPointerException: The installFolder parameter cannot be null
	at org.openide.util.Parameters.notNull(Parameters.java:64)
	at org.netbeans.modules.java.j2seplatform.platformdefinition.J2SEPlatformFactory.create(J2SEPlatformFactory.java:51)
	at org.netbeans.modules.gradle.java.spi.support.JavaToolchainSupport.detectPlatform(JavaToolchainSupport.java:87)
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1228)
	at org.netbeans.modules.gradle.java.spi.support.JavaToolchainSupport.platformByHome(JavaToolchainSupport.java:70)
	at org.netbeans.modules.gradle.java.classpath.BootClassPathImpl.createPath(BootClassPathImpl.java:59)
	at org.netbeans.modules.gradle.java.classpath.AbstractGradleClassPathImpl.getResources(AbstractGradleClassPathImpl.java:108)
	at org.netbeans.api.java.classpath.ClassPath.entries(ClassPath.java:363)
	

[mbien]

code looks good. Haven't tested it though.