@Test
	public void testGenerateChildren_ReadReference_NullInstruction() throws Exception {

		//
		// Have a reference in the function to a place that is not another function, and the
		// reference is a read reference.
		// Note: since we did not have the builder put an instruction at the 'to' address,
		//       the instruction there is null.
		//

		builder.createMemoryReference(firstCalledFunctionAddress, "0x1000", RefType.READ,
			SourceType.USER_DEFINED);
		List<GTreeNode> children = node1.generateChildren(TaskMonitor.DUMMY);
		assertTrue(children.isEmpty());

	}
This test class is covering the various types of 'calls' a function can make, such as a
direct call instruction, or through a user-defined reference.   External calls are also a bit
tricky.   This test was created by following the code paths in {@link OutgoingCallNode}
and making sure that each path was followed.
Ghidra/Features/Base/src/test.slow/java/ghidra/app/plugin/core/calltree/OutgoingCallNodeTest.java