bazel: Crash when building `objc_library`

Description of the problem / feature request:

2021/08/17 20:46:00 Using unreleased version at commit cfc2d1d98f827787b80d9c211fc17f97b03c558b
Starting local Bazel server and connecting to it...
Analyzing: target //some:objc_target (40 packages loaded, 537 targets configured)
    currently loading: @bazel_tools//src/tools/launcher
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'ConfiguredTargetKey{label=//some:objc_target, config=BuildConfigurationValue.Key[759ce837fa28c3e3aaa73b2dc2f899ac4be3297e9cf4724d9361269d6cba6b4b]}' (requested by nodes )
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:644)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:382)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)
Caused by: net.starlark.java.eval.Starlark$UncheckedEvalException: NullPointerException thrown during Starlark evaluation (//some:objc_target)
	at <starlark>.create_compilation_attributes(<builtin>:0)
	at <starlark>._build_common_variables(/virtual_builtins_bzl/common/objc/compilation_support.bzl:79)
	at <starlark>._objc_library_impl(/virtual_builtins_bzl/common/objc/objc_library.bzl:114)
Caused by: java.lang.NullPointerException
	at java.base/java.lang.String.replace(Unknown Source)
	at com.google.devtools.build.lib.rules.objc.ObjcStarlarkInternal.expandFlag(ObjcStarlarkInternal.java:142)
	at com.google.devtools.build.lib.rules.objc.ObjcStarlarkInternal.expandToolchainAndRuleContextVariables(ObjcStarlarkInternal.java:116)
	at com.google.devtools.build.lib.rules.objc.ObjcStarlarkInternal.createCompilationAttributes(ObjcStarlarkInternal.java:82)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.base/java.lang.reflect.Method.invoke(Unknown Source)
	at net.starlark.java.eval.MethodDescriptor.call(MethodDescriptor.java:162)
	at net.starlark.java.eval.BuiltinFunction.fastcall(BuiltinFunction.java:77)
	at net.starlark.java.eval.Starlark.fastcall(Starlark.java:619)
	at net.starlark.java.eval.Eval.evalCall(Eval.java:672)
	at net.starlark.java.eval.Eval.eval(Eval.java:489)
	at net.starlark.java.eval.Eval.execAssignment(Eval.java:109)
	at net.starlark.java.eval.Eval.exec(Eval.java:268)
	at net.starlark.java.eval.Eval.execStatements(Eval.java:82)
	at net.starlark.java.eval.Eval.execFunctionBody(Eval.java:66)
	at net.starlark.java.eval.StarlarkFunction.fastcall(StarlarkFunction.java:191)
	at net.starlark.java.eval.Starlark.fastcall(Starlark.java:619)
	at net.starlark.java.eval.Eval.evalCall(Eval.java:672)
	at net.starlark.java.eval.Eval.eval(Eval.java:489)
	at net.starlark.java.eval.Eval.execAssignment(Eval.java:109)
	at net.starlark.java.eval.Eval.exec(Eval.java:268)
	at net.starlark.java.eval.Eval.execStatements(Eval.java:82)
	at net.starlark.java.eval.Eval.execFunctionBody(Eval.java:66)
	at net.starlark.java.eval.StarlarkFunction.fastcall(StarlarkFunction.java:191)
	at net.starlark.java.eval.Starlark.fastcall(Starlark.java:619)
	at com.google.devtools.build.lib.analysis.starlark.StarlarkRuleConfiguredTargetUtil.buildRule(StarlarkRuleConfiguredTargetUtil.java:98)
	at com.google.devtools.build.lib.analysis.ConfiguredTargetFactory.createRule(ConfiguredTargetFactory.java:368)
	at com.google.devtools.build.lib.analysis.ConfiguredTargetFactory.createConfiguredTarget(ConfiguredTargetFactory.java:188)
	at com.google.devtools.build.lib.skyframe.SkyframeBuildView.createConfiguredTarget(SkyframeBuildView.java:1040)
	at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.createConfiguredTarget(ConfiguredTargetFunction.java:981)
	at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.compute(ConfiguredTargetFunction.java:368)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:560)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:382)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)

Bugs: what’s the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I’ll submit an example later.

What operating system are you running Bazel on?

macOS

What’s the output of bazel info release?

I used last_green Bazel.