Hey, I'm just using this example: http://www.minecraftforge.net/wiki/Custom_2D_Inventory_Item_Renderer
Also it's not running on server, since I'm connecting to my server by IP and it is still crashing.
Maybe it's helpful, a full error stack trace with the function commented out. Then Java can't find other methods or instances:
public void renderItem(ItemRenderType type, ItemStack itemStack, Object... data) {
System.out.println("renderItem!!!");
// ====================== Render item texture ======================
//Icon icon = itemStack.getIconIndex();
//renderItem.renderIcon(0, 0, icon, 16, 16);
// ====================== Render OpenGL square shape ======================
GL11.glDisable(GL11.GL_TEXTURE_2D);
GL11.glEnable(GL11.GL_BLEND);
GL11.glDepthMask(false);
GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
Tessellator tessellator = Tessellator.instance;
tessellator.startDrawing(GL11.GL_QUADS);
tessellator.setColorRGBA(0, 0, 0, 128);
tessellator.addVertex(0, 0, 0);
tessellator.addVertex(0, 8, 0);
tessellator.addVertex(8, 8, 0);
tessellator.addVertex(8, 0, 0);
tessellator.draw();
GL11.glDepthMask(true);
GL11.glDisable(GL11.GL_BLEND);
// ====================== Render text ======================
GL11.glEnable(GL11.GL_TEXTURE_2D);
String text = Integer.toString(itemStack.getItemDamage());
/*
FontRenderer fontRenderer = Minecraft.getMinecraft().fontRenderer;
fontRenderer.drawStringWithShadow(text, 1, 1, 0xFFFFFF);
*/
}