fix nil pointer exception (gardener#969)

docs/guided-tour/blueprints/helm-chart-resource/README.md

@@ -14,6 +14,9 @@ Using this oci reference, the landscaper is able to fetch the helm chart and dep
 
 ## Referencing the Helm Chart with a Component
 
+>**_CAUTION_**: Specifying the *Repository Context* directly in the *Installation* is **deprecated**. In order to be able to
+> use this feature, the *Repository Context* has to be specified in the Landscaper *[Context](../../../usage/Context.md)*. 
+
 In the [previous section](../external-blueprint/README.md), the concept of components was introduced, as an alternative
 means to reference the blueprints in the installation instead of having to write the blueprints directly inline into the
 installation (as it was done in the [first several examples](../../hello-world/installation/installation.yaml)). 

pkg/deployer/helm/chartresolver/chart.go

@@ -81,6 +81,14 @@ func getChartFromResourceRef(ctx context.Context, resourceRef string, lsCtx *lsv
 
 	octx := ocm.New(datacontext.MODE_EXTENDED)
 
+	if lsCtx == nil {
+		return nil, fmt.Errorf("landscaper context cannot be nil")
+	}
+	if lsCtx.RepositoryContext == nil || lsCtx.RepositoryContext.Raw == nil {
+		return nil, fmt.Errorf("landscaper context %s/%s does not specify a repository context but has"+
+			" to specify a repository context to resolve resource from an ocm reference", lsCtx.Namespace, lsCtx.Name)
+	}
+
 	// Credential Handling
 	// resolve all credentials from registry pull secrets
 	registryPullSecretRefs := lib.GetRegistryPullSecretsFromContext(lsCtx)