ADDITIONAL SYSTEM INFORMATION :
JDK: Eclipse Adoptium JDK-21.0.3.9-Hotspot

A DESCRIPTION OF THE PROBLEM :
In a ternary conditional construction, if the return value might either be a primitive long or null, the compiler does not throw an error.

A primitive long and null are incompatible. Hence, it is expected that the behavior of the compiler is similar to how it would behave if the return value might have been "a primitive long or a string". In case of "a primitive long or a string", the compiler throws an error like:
>>
java: incompatible types: bad type in conditional expression
    bad type in conditional expression
      java.lang.String cannot be converted to java.lang.Long
<<

Example code to reproduce the issue:
>>
Object a = null;
final Long b = a instanceof Long ? Long.class.cast(a).longValue() : a instanceof Integer ? Integer.class.cast(a).longValue() : null;
<<

The example throws a
>>
java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()"
<<
on runtime.