48.0 NPE on designating a hex with TAG for incoming Homing Arrow IV

[gsparks3]

Environment

Standalone Megamek 48.0
Windows 10
Java 11.0.10

Description

During a campaign game, we encountered a game-stall-inducing NPE on designating a hex with TAG with an incoming Homing Arrow IV round. Log files and a save that allows reproduction are provided.

Steps to reproduce:

1. Start the saved game as player "sprero"
2. Target a ground hex within 8 hexes of the nearby marker for the incoming Homing AIV round and hit "Done".
3. Observe the game stall and NPE in the log.

Files

megameklog.txt
homingAIVtagCrash.sav.gz

[HammerGS]

megamek.common.HexTarget cannot be cast to class megamek.common.Entity (megamek.common.HexTarget and megamek.common.Entity are in unnamed module of loader 'app') at megamek.common.weapons.WeaponHandler.calcCounterAV(WeaponHandler.java:249) at megamek.common.weapons.ArtilleryWeaponIndirectHomingHandler.handleAMS(ArtilleryWeaponIndirectHomingHandler.java:472) at megamek.common.weapons.ArtilleryWeaponIndirectHomingHandler.handle(ArtilleryWeaponIndirectHomingHandler.java:207) at megamek.server.Server.handleAttacks(Server.java:35528) at megamek.server.Server.handleAttacks(Server.java:35457) at megamek.server.Server.endCurrentPhase(Server.java:2947) at megamek.server.Server.changeToNextTurn(Server.java:3202) at megamek.server.Server.endCurrentTurn(Server.java:2212) at megamek.server.Server.receiveAttack(Server.java:13429) at megamek.server.Server.handle(Server.java:31381) at megamek.server.Server$PacketPump.run(Server.java:254) at java.base/java.lang.Thread.run(Thread.java:834)

[sixlettervariables]

It looks like we might could just exclude non-entities here:

megamek/megamek/src/megamek/common/weapons/WeaponHandler.java

Lines 242 to 249 in 1dba185

	protected int calcCounterAV() {
	if (!checkPDConditions()) {
	return 0;
	}
	int counterAV = 0;
	int amsAV = 0;
	double pdAV = 0;
	Entity entityTarget = (Entity) target;

Not familiar enough with that code block to suggest it though.

[sixlettervariables]

Proximate cause is the base checkPDConditions excludes non-entities, but ArtilleryWeaponIndirectHomingHandler overrides and does not:

megamek/megamek/src/megamek/common/weapons/ArtilleryWeaponIndirectHomingHandler.java

Lines 424 to 434 in 1dba185

	/**
	* Checks to see if the basic conditions needed for point defenses to work are in place
	* Artillery weapons need to change this slightly
	*/
	protected boolean checkPDConditions() {
	advancedPD = game.getOptions().booleanOption(OptionsConstants.ADVAERORULES_STRATOPS_ADV_POINTDEF);
	if ((target == null) || !advancedPD) {
	return false;
	}
	return true;
	}