Since the release of 0.11.0 when I call 'Run Test' I get 'Failed to run tests' and a message window saying 'Error'.
The repo I've been working with is here: https://github.com/angelozerr/lsp4xml
And tests have been working up until the new release.

Try lsp4xml/org.eclipse.lsp4xml/src/test/java/org/eclipse/lsp4xml/services/XMLFormatterTest.java to run a test.
@NikolasKomonen Thanks for reporting. I'll take a look.
The main cause is that in the code here: https://github.com/Microsoft/vscode-java-test/blob/master/com.microsoft.java.test.plugin/src/main/java/com/microsoft/java/test/plugin/util/RuntimeClassPathUtils.java#L42
We need to check the project nature as well instead of only doing the null check.
Java Test Runner 0.11.1 has been released.
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The main cause is that in the code here: https://github.com/Microsoft/vscode-java-test/blob/master/com.microsoft.java.test.plugin/src/main/java/com/microsoft/java/test/plugin/util/RuntimeClassPathUtils.java#L42
We need to check the project nature as well instead of only doing the null check.