Angular library build error: TypeError: Cannot read property 'type' of null

In Angular v6, when attempting to build my library I am running into an extremely non-descriptive BUILD ERROR Cannot read property 'type' of null

BUILD ERROR
Cannot read property 'type' of null
TypeError: Cannot read property 'type' of null
    at /Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:15378:27
    at Array.forEach (<anonymous>)
    at removeSummaryDuplicates (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:15377:11)
    at TemplateParser.tryParseHtml (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:14806:34)
    at TemplateParser.tryParse (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:14799:21)
    at TemplateParser.parse (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:14780:27)
    at AotCompiler._parseTemplate (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:20483:43)
    at AotCompiler._createTypeCheckBlock (/Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:20250:23)
    at /Users/John/apps/tests/service-work/node_modules/@angular/compiler/bundles/compiler.umd.js:20220:27
    at Array.forEach (<anonymous>)

Having already solved this problem, I'm making this post to help anyone else running into this issue.

Solution

Update

It seems like my diagnosis of this issue was incorrect (it's been so long now I can't remember exactly how I fixed it). Check out this issue in the Angular repo for what sounds like the correct diagnosis,

Original answer:

So after a LOT of debugging, I figured it out:

My custom library, let call it library A, imports another custom library, library B, into library A's NgModule. Library B exports several components and modules from it's main NgModule. The problem was that, while library B exported the components and modules from it's NgModule, I failed to also export those components and modules via javascript/typescript. The solution was to make sure to export any components / modules via typescript that I exported in the NgModule.

Example problem code

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { LibraryBComponent } from './library-b.component';

@NgModule({
  imports: [CommonModule],
  declarations: [LibraryBComponent],
  exports: [LibraryBComponent],
})
export class LibraryBModule {}

Solution code

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { LibraryBComponent } from './library-b.component';

export { LibraryBComponent }      // <-- addition

@NgModule({
  imports: [CommonModule],
  declarations: [LibraryBComponent],
  exports: [LibraryBComponent],
})
export class LibraryBModule {}