How to generate Random number based on range within List size without accessing zero

I have an Ant Colony Simulator. It is a 27 x 27 grid, and there is a Forager Ant class which locates food and the highest pheromone levels. I need to randomly generate the movement within a range.

This is a very large project, so here is only the method in question (if that's enough):

    private GridNode locateHighestPherms() {
    Random randomNode = new Random();
    LinkedList<GridNode> neighborNodeList = gridLocation.getNeighboringNodes(); //a List of Node Objects that keeps track of adjacent nodes
    LinkedList<GridNode> randomGridNode = new LinkedList<>(); //random destination Node

    for(Iterator<GridNode> gridNodeIterator = neighborNodeList.iterator(); gridNodeIterator.hasNext();) {
        GridNode alreadyVisited = gridNodeIterator.next();
        if(foragerMoveHistory.contains(alreadyVisited) || !alreadyVisited.isVisible()) {
            gridNodeIterator.remove();
        }
    }
    if(neighborNodeList.size() == 0) {
        neighborNodeList = gridLocation.getNeighboringNodes();
    }
    GridNode nodeWithMostPherms = neighborNodeList.get(0);

    for(int checkNode = 1; checkNode < neighborNodeList.size(); checkNode++) {
        if(nodeWithMostPherms.isVisible() && nodeWithMostPherms.getPheromoneUnit() < neighborNodeList.get(checkNode).getPheromoneUnit()) {
            nodeWithMostPherms = neighborNodeList.get(checkNode);
        }
    }
    for (GridNode neighborNode : neighborNodeList) {
        if ((neighborNode.getPheromoneUnit() == nodeWithMostPherms.getPheromoneUnit()) && neighborNode.isVisible()) {
            randomGridNode.add(neighborNode);
        }
    }
    //DEBUGGING
    //System.out.println(randomGridNode.size());
    nodeWithMostPherms = randomGridNode.get(randomNode.nextInt(randomGridNode.size()));
    //nodeWithMostPherms = randomGridNode.get(RandomInstance.randomNumberGen(1, randomGridNode.size()));
    return nodeWithMostPherms;
  }
}

Right there ^ the assignment to nodeWithMostPherms is where I need to access the next Random number. However, when I originally tried the code that's commented out, it was crashing because at times I was trying to access zero when the list size was zero.

I will show you my RandomInstance class. It's short and sweet:

    import java.util.Random;

public class RandomInstance {

static int randomNumber;

public static int randomNumberGen(int lowRange, int highRange) {

    Random numberGenerator = new Random(); //I would prefer not to have this.
    randomNumber = numberGenerator.nextInt(highRange - lowRange + 1) + lowRange;

    /** EXAMPLE FOR REFERENCE
     * setFoodUnitAmount(RandomInstance.randomNumberGen(500, 1000));
     */
    return randomNumber;
  }
}

The reason I have my own Random class is because there are many instances of random numbers that are generated, and it was suggested to create our own so we don't have a bunch of instances of java.util.Random all over the place.

Does anyone have any suggestions on how I can make this fit my RandomInstance class?