SpelExpressionParser throws IllegalStateException instead of ParseException for invalid expression

[andre-aktivconsultancy]

Environment Details
Spring Framework Version: 6.0.6
Spring Boot Version: 3.0.4
Java Version: 17

Description
According to the Spring documentation, invoking parseExpression on a SpelExpressionParser should only result in a ParseException when provided with an invalid expression. However, in practice, it throws an IllegalStateException with the message "No node".

Steps to Reproduce

• Create a new SpelExpressionParser instance.
• Attempt to parse an invalid expression, e.g., ")".
• Observe the exception thrown.

Test Code

@Test
void parse_invalidExpression_shouldThrowParseException() {
    final var expressionParser = new SpelExpressionParser();
    final var invalidExpression = ")";
    
    final var thrown = catchThrowable(() -> expressionParser.parseExpression(invalidExpression));

    assertThat(thrown).isInstanceOf(ParseException.class);
}

Expected Behavior
The test should pass, with ParseException being thrown for the invalid expression.

Actual Behavior
The test fails with the following output:

java.lang.AssertionError: 
Expecting actual throwable to be an instance of:
  org.springframework.expression.ParseException
but was:
  java.lang.IllegalStateException: No node
	at org.springframework.util.Assert.state(Assert.java:76)
	at org.springframework.expression.spel.standard.InternalSpelExpressionParser.doParseExpression(InternalSpelExpressionParser.java:132)
	at org.springframework.expression.spel.standard.SpelExpressionParser.doParseExpression(SpelExpressionParser.java:61)

[jhoeller]

This is actually a regression from our nullability efforts in 5.0 where we introduced additional state assertions. We're going to fix this through refining those assertions, letting the original parse exceptions be arrived at again.