[1.7.2] Button's action in custom GUI keep crashing the server

I created a block with it's own GUI. What the GUI dose is that creates an explosion based on number you input in the text box. Everything works on singleplayer, but on multiplayer, the server crashes when i click on the button

Here's the method

protected void actionPerformed(GuiButton button) {

World world = null;

WorldServer[] list = MinecraftServer.getServer().worldServers; // in console it says this line crashes

for (WorldServer ins : list) {

if (ins.provider.dimensionId == entity.worldObj.provider.dimensionId)

world = ins;

}

if (world == null)

world = list[0];

if (button.id == 0) {

if (true) {

if (power.getText().isEmpty())

power.setText("");

else if (power.getText().matches("[a-zA-Z]+"))

power.setText("");

else if (power.getText().matches("[0-9]+")) {

this.mc.displayGuiScreen((GuiScreen) null);

world.createExplosion((Entity) null, i, j - 10, k, Integer.parseInt(power.getText().trim()), true);

}

else

power.setText("");

}

}

}

How can I do that? I never mess around with packets before, I heard, but don't know what it is.

Alright. Thank you for leading me on what is causing the problem