I've gone and read through a lot of these forums but I find it difficult to apply the advice to my code, so here I am posting!

I get the following error from my Test class for my Trigger:

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, updateRoomLocation: execution of BeforeInsert

caused by: System.NullPointerException: Attempt to de-reference a null object

Trigger.updateRoomLocation: line 30, column 1: []

 It is the same error for all three test functions. The test class is below, and the trigger below that:

@isTest
private class TestRoomBuildingAssignment {
    public static Location__c createLocation(Boolean createRoom){
        Location__c location = new Location__c(
            caan__c = '1000',
            Building_Code__c = 'TEST',
            Location_Name__c = 'Test building'
        );
        if(createRoom == True){
            location.Room__c = '100';
        }
        insert location;
        return location;
    }
    static Location__c createLocation(){
        return createLocation(False);
    }
    static testMethod void testRoomAssignment() {
        test.startTest();
        Location__c location = createLocation();
        Location__c room = createLocation(True);
        test.stopTest();
        
        system.assertEquals(room.Location__c, location.Id);
    }
    static testMethod void testLocationName() {
        //  relies on createLocation making a location with
        //  caan__c = 1000, Building_Code__c = 'TEST' & Room__c = 100
        
        test.startTest();
        Location__c location = createLocation();
        test.stopTest();
        
        system.assertEquals(location.Name, '0001-TEST');
    }
    static testMethod void testRoomName() {
        //  relies on createLocation making a location with
        //  caan__c = 1000, Building_Code__c = 'TEST' & Room__c = 100
        
        test.startTest();
        Location__c location = createLocation(True);
        test.stopTest();
        
        system.assertEquals(location.Name, '0001-TEST-100');
    }
}

 Trigger:

trigger updateRoomLocation on Location__c (before insert) {
    /*
        We want to update all rooms with their correspondent buildings. We do this by:
        1.  getting all caan IDs for our trigger set
        2.  finding all buildings who have that caan id (buildings have no rooms)
        3.  matching room ID with building ID
    */

    // we match rooms to caan numbers
    Map<Id, String> roomMap = new Map<Id, String>();
    
    // eventually we match room Ids to 
    Map<Id, Location__c> updateMap = new Map<Id, Location__c>();
    
    for (Location__c room : Trigger.new){
        roomMap.put(room.Id, room.caan__c);
    }

    if (roomMap.size() > 0){
        Map<String, Location__c> buildingMap = new Map<String, Location__c>([
                SELECT caan__c, Id FROM Location__c 
                WHERE caan__c IN :roomMap.values()
                AND Room__c = ''
        ]);
        for (String roomId : roomMap.keySet()){
            String caan = roomMap.get(roomId);
            updateMap.put(roomId, buildingMap.get(caan));
        }
        for (Location__c room : Trigger.new){
            room.Location__c = updateMap.get(room.Id).Id;
        }
    }
}

Any help is greatly appreciated!