terraform: Handle missing source in nodeInstallModule

internal/terraform/node_module_install.go

@@ -73,6 +73,12 @@ func (n *nodeInstallModule) References() []*addrs.Reference {
 func (n *nodeInstallModule) Execute(ctx EvalContext, walkOp walkOperation) tfdiags.Diagnostics {
 	var diags tfdiags.Diagnostics
 
+	if n.ModuleCall.SourceExpr == nil {
+		// Cannot install a module without a source
+		// Outer diags should already contain a parsing error
+		return diags
+	}
+
 	var version configs.VersionConstraint
 	hasVersion := false
 	if n.ModuleCall.VersionExpr != nil {
@@ -141,6 +147,12 @@ func (n *nodeInstallModule) DynamicExpand(ctx EvalContext) (*Graph, tfdiags.Diag
 	var g Graph
 	var diags tfdiags.Diagnostics
 
+	if n.Config == nil {
+		// Cannot expand without a config. This can happen when something goes wrong
+		// during module installation/Execute() above.
+		return nil, diags
+	}
+
 	expander := ctx.InstanceExpander()
 	_, call := n.Addr.Call()
 	expander.SetModuleSingle(n.Path(), call)