Crash when loading custom GUI

[The Typholorian]

Caused by: java.lang.NullPointerException: Cannot read field "level" because "this.minecraft" is null

This is my first big error when coding. I looked it up, but there are a bunch of references to Dawncraft and not enough to figure out what was going wrong. I found this, but it involves a command and my GUI only opens when you shift + right-click while holding an item. They fixed it by not calling it with a command, but I am not using a command. Here's the render of my menu, which is the only part that changed between it working and not:
 

@Override
public void render(@Nonnull PoseStack mstack, int mouseX, int mouseY, float partialTicks) {
int midx = this.width / 2;
int midy = this.height / 2;

this.renderBackground(mstack);
drawCenteredString(mstack, font, I18n.get("config.shape_tool"), midx, 4, 0xC3D1D8);

Utils.renderImage(midx - 12, midy, 20, 20, "textures/gui/plus.png", mstack);
addRenderableWidget(new Button(midx - 12, midy, 10, 10, Component.translatable(""), this::increaseax));
Utils.renderImage(midx + 12, midy, 20, 20, "textures/gui/minus.png", mstack);
addRenderableWidget(new Button(midx + 12, midy, 10, 10, Component.translatable(""), this::decreaseax));
drawCenteredString(mstack, font, String.valueOf(shapetoolax), midx, midy + 10, 0xC3D1D8);

super.render(mstack, mouseX, mouseY, partialTicks);
}

Utils.renderImage is this:
 

public static void renderImage(int x, int y, int width, int height, String img, PoseStack mstack) {
RenderSystem.setShaderColor(1, 1, 1, 1);
RenderSystem.enableBlend();
RenderSystem.defaultBlendFunc();
RenderSystem.setShaderTexture(0, new ResourceLocation(PNegative.MODID, img));
blit(mstack, x, y, 0, 0, width, height, width, height);
RenderSystem.disableBlend();
}

I have used it before and it has worked, so it can't be a problem. 

[The Typholorian]

Update: I restarted the game and it was working, but when I closed the menu and re-opened it it crashed with error message Caused by: java.net.UnknownHostException: sessionserver.mojang.com. 

[warjort]

Your question is pretty much unanswerable. You don't show all the relevant code and don't even post the full error message.

 

Don't post code incomplete snippets in the forum. Put a minimal project on github that reproduces your problem that we can look at and run for ourselves.

Or just your full project with instructutions on how to reproduce the issue and a pointer to relevant code.

 

Guess (I shouldn't have to guess):

Quote

Caused by: java.lang.NullPointerException: Cannot read field "level" because "this.minecraft" is null

This error looks like you caching something in a static field that has now become invalid. Or otherwise reusing an object that is no longer valid.

Don't put stuff in static fields unless it is effectively immutable.

Doing so will at best give you weird bugs. At worst it will cause memory leaks and data corruption.