tns preview fails when local plugin is referenced with tag in package.json

[sis0k0]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 5.0.0-2018-10-11-12463
• Cross-platform modules: rc

Describe the bug
If the version of some plugin in the package.json is specified with tag (f.e. rc) instead of version (f.e. 5.0.0), the tns preview command fails with:

TypeError: Cannot read property 'major' of null
    at PreviewAppPluginsService.getWarningForPluginCore (/home/sis0k0/nativescript-cli/lib/services/livesync/playground/preview-app-plugins-service.ts:82:30)

The PreviewAppPluginsService uses semver to check some stuff with the versions of the plugins in the package.json. However, semver.coerce('rc') returns null and breaks the following logic.

nativescript-cli/lib/services/livesync/playground/preview-app-plugins-service.ts

Lines 74 to 91 in b9bfc68

	private getWarningForPluginCore(localPlugin: string, localPluginVersion: string, devicePluginVersion: string, deviceId: string): string {
	this.$logger.trace(`Comparing plugin ${localPlugin} with localPluginVersion ${localPluginVersion} and devicePluginVersion ${devicePluginVersion}`);
	if (devicePluginVersion) {
	const localPluginVersionData = semver.coerce(localPluginVersion);
	const devicePluginVersionData = semver.coerce(devicePluginVersion);
	if (localPluginVersionData.major !== devicePluginVersionData.major) {
	return util.format(PluginComparisonMessages.LOCAL_PLUGIN_WITH_DIFFERENCE_IN_MAJOR_VERSION, localPlugin, localPluginVersion, devicePluginVersion);
	} else if (localPluginVersionData.minor > devicePluginVersionData.minor) {
	return util.format(PluginComparisonMessages.LOCAL_PLUGIN_WITH_GREATHER_MINOR_VERSION, localPlugin, localPluginVersion, devicePluginVersion);
	}
	return null;
	}
	return util.format(PluginComparisonMessages.PLUGIN_NOT_INCLUDED_IN_PREVIEW_APP, localPlugin, deviceId);
	}

To Reproduce

1. Create a hello-world app.
2. Add this dependency to your package.json:

...
"dependencies": {
	"tns-core-modules": "rc",
	...
}

3. Run tns preview and scan the barcode.

Expected behavior
To ignore the above check if the tag is beta, rc or next.
I can submit a PR if you agree that's the correct behaviour.

[Fatme]

We discussed the problem personally and decided to skip the check if the version is not valid semver version. The same approach is used here: https://github.com/NativeScript/nativescript-cli/blob/master/lib/helpers/bundle-validator-helper.ts#L27