[FEM] improve new default solver setting

- fix bug if no solver should be default
- only provide solvers that are available to be the default
- also add commit opened transactions
- also remove unused include and correct an include

src/Mod/Fem/Gui/DlgSettingsFemGeneralImp.cpp

@@ -24,6 +24,8 @@
 
 #include "PreCompiled.h"
 
+#include <App/Application.h>
+
 #include "DlgSettingsFemGeneralImp.h"
 #include "ui_DlgSettingsFemGeneral.h"
 
@@ -35,6 +37,37 @@ DlgSettingsFemGeneralImp::DlgSettingsFemGeneralImp(QWidget* parent)
     , ui(new Ui_DlgSettingsFemGeneralImp)
 {
     ui->setupUi(this);
+
+    // fill solvers combo with available solvers
+    ui->cmb_def_solver->clear();
+    std::vector<std::string> Solvers = {"None"};
+
+    ParameterGrp::handle hGrp = App::GetApplication().GetParameterGroupByPath(
+        "User parameter:BaseApp/Preferences/Mod/Fem/Ccx");
+    auto ccxBinaryPath = hGrp->GetASCII("ccxBinaryPath", "");
+    if (!ccxBinaryPath.empty())
+        Solvers.push_back("CalculiX");
+    hGrp = App::GetApplication().GetParameterGroupByPath(
+        "User parameter:BaseApp/Preferences/Mod/Fem/Elmer");
+    auto elmerBinaryPath = hGrp->GetASCII("elmerBinaryPath", "");
+    if (!elmerBinaryPath.empty())
+        Solvers.push_back("Elmer");
+    hGrp = App::GetApplication().GetParameterGroupByPath(
+        "User parameter:BaseApp/Preferences/Mod/Fem/mystran");
+    auto MystranBinaryPath = hGrp->GetASCII("MystranBinaryPath", "");
+    if (!MystranBinaryPath.empty())
+        Solvers.push_back("Mystran");
+    hGrp = App::GetApplication().GetParameterGroupByPath(
+        "User parameter:BaseApp/Preferences/Mod/Fem/Z88");
+    auto z88BinaryPath = hGrp->GetASCII("z88BinaryPath", "");
+    if (!z88BinaryPath.empty())
+        Solvers.push_back("Z88");
+
+    QStringList solversList;
+    for (auto item : Solvers) {
+        solversList << QLatin1String(item.c_str());
+    }
+    ui->cmb_def_solver->addItems(solversList);
 }
 
 DlgSettingsFemGeneralImp::~DlgSettingsFemGeneralImp()