public static void checkState(boolean expression, @Nullable Object errorMessage) {
		if (!expression) {
			throw new IllegalStateException(String.valueOf(errorMessage));
		}
	}
Ensures the truth of an expression involving the state of the calling instance, but not involving any parameters
to the calling method.

@param expression   a boolean expression
@param errorMessage the exception message to use if the check fails; will be converted to a string using
                    {@link String#valueOf(Object)}
ngrinder-core/src/main/java/org/ngrinder/common/util/Preconditions.java