Describe the bug
Polyglot sampler agent sometimes throws the following NPE:

java.lang.NullPointerException
	at com.oracle.truffle.tools.profiler.impl.CPUSamplerInstrument.getSampler(CPUSamplerInstrument.java:93)
	at com.oracle.truffle.tools.profiler.CPUSampler.find(CPUSampler.java:270)
	at org.graalvm.visualvm.sampler.truffle.stagent.Truffle.getAllStackTracesInstances(Truffle.java:216)
	at org.graalvm.visualvm.sampler.truffle.stagent.Truffle.dumpAllThreads(Truffle.java:89)

To Reproduce
Steps to reproduce the behavior:

1. start VisualVM
2. open GraalVM application
3. try to use polyglot sampler, click on 'CPU' button
4. NPE is thrown to console of the sampled application, VisualVM displays no CPU results

Desktop (please complete the following information):

• OS: macOS
• JDK version : JDK 1.8.0_181
• Version Dev.