Compiler crashes when diverting to choice or gather labels with parameters

[benabutler]

If an ink file attempts to pass an argument while diverting to a choice or gather label, the compiler will crash with this stack:

Ink Compiler threw exception 
Error: Object reference not set to an instance of an object
---- Trace ----
  at Ink.Parsed.Divert.GenerateRuntimeObject () [0x000cb] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Divert.cs:90 
  at Ink.Parsed.Object.get_runtimeObject () [0x00012] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Object.cs:55 
  at Ink.Parsed.Weave.GenerateRuntimeObject () [0x0009b] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Weave.cs:198 
  at Ink.Parsed.Object.get_runtimeObject () [0x00012] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Object.cs:55 
  at Ink.Parsed.FlowBase.GenerateRuntimeObject () [0x0019f] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\FlowBase.cs:247 
  at Ink.Parsed.Object.get_runtimeObject () [0x00012] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Object.cs:55 
  at Ink.Parsed.FlowBase.GenerateRuntimeObject () [0x000eb] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\FlowBase.cs:217 
  at Ink.Parsed.Object.get_runtimeObject () [0x00012] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Object.cs:55 
  at Ink.Parsed.Story.ExportRuntime (Ink.ErrorHandler errorHandler) [0x0013b] in Assets\Ink\InkLibs\InkCompiler\ParsedHierarchy\Story.cs:155 
  at Ink.Compiler.Compile () [0x00085] in Assets\Ink\InkLibs\InkCompiler\Compiler.cs:53 
  at Ink.UnityIntegration.InkCompiler.CompileInkThreaded (System.Object itemObj) [0x00085] in Assets\Ink\Editor\Core\Compiler\InkCompiler.cs:576 
--------

It looks to me like the Divert.GenerateRuntimeObject assumes targetContent will be a FlowBase if the user is providing arguments, but gathers and choices are just regular Ink.Parsed.Object objects. I don't think passing arguments to a gather point is valid Ink syntax - I only hit the crash because I mistyped something - but the compiler should probably provide a nice error message like it does for diverting with parameters to knots that don't take parameters.

Here's a repro demonstrating the behavior:
InkCompilerCrashRepro.txt

I found this issue on v1.2.0 while using the ink-unity-integration plugin v1.2.1, but also reproduced it on a standalone build of inky v1.1.0. Originally filed in the unity plugin repo here: inkle/ink-unity-integration#208